…7856)

### What problem does this PR solve?
Problem Summary:

Iceberg columns declared `NOT NULL` are reported as nullable by `DESC`,
`SHOW CREATE TABLE`, `SHOW [FULL] COLUMNS`, and
`information_schema.columns`. For example, `id BIGINT NOT NULL` appears
as `Null: Yes` / `IS_NULLABLE: YES` or `bigint NULL`, even though
inserting NULL is rejected.

Build independent display columns from one retained Iceberg metadata
generation and preserve their top-level required/optional flags. These
metadata-only paths use the display schema, including `SHOW COLUMNS`
with `WHERE` through `information_schema.columns`. Metadata enumeration
retains per-table schema-load failure handling. Scan columns keep their
existing nullable semantics for historical files and schema evolution;
nested-type and write-validation behavior is preserved.

### Release note

Correct top-level Iceberg column nullability in `DESC`, `SHOW CREATE
TABLE`, `SHOW [FULL] COLUMNS`, and `information_schema.columns`.

### Check List (For Author)

- Test
    - [x] Regression test
    - [x] Unit Test
    - [ ] Manual test (add detailed scripts or steps below)
    - [ ] No need to test or manual test. Explain why:
- [ ] This is a refactor/code format and no logic has been changed.
        - [ ] Previous test can cover this change.
        - [ ] No code files have been changed.
        - [ ] Other reason
- Behavior changed:
    - [ ] No.
    - [x] Yes. 

- Does this need documentation?
    - [x] No.
    - [ ] Yes.

### Check List (For Reviewer who merge this PR)

- [ ] Confirm the release note
- [ ] Confirm test cases
- [ ] Confirm document
- [ ] Add branch pick label

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

Related PR: apache#67856

Problem Summary:

Iceberg columns declared NOT NULL are shown as nullable by DESC, SHOW CREATE
TABLE, SHOW [FULL] COLUMNS and information_schema.columns. The connector reads
every top-level column as nullable on purpose, since old data files can miss a
column, and these commands reuse that read schema.

ConnectorColumn now also carries the nullability the source declares. The
Iceberg connector sets it from the field's required flag, and these metadata
commands use it. The columns used for reads and writes stay nullable.

Show the declared nullability of Iceberg columns in DESC, SHOW CREATE TABLE,
SHOW COLUMNS and information_schema.columns.

- Test: Regression test, Unit Test
- Behavior changed: Yes. Required Iceberg columns are now shown as NOT NULL.
- Does this need documentation: No

### What problem does this PR solve?

Related PR: #67856

Problem Summary:

Iceberg columns declared `NOT NULL` are shown as nullable by `DESC`,
`SHOW CREATE TABLE`, `SHOW [FULL] COLUMNS` and
`information_schema.columns`. The connector reads every top-level column
as nullable on purpose, since old data files can miss a column, and the
display paths reuse that read schema.

`ConnectorColumn` now also carries the nullability the source declares.
The Iceberg connector sets it from the field's required flag, and these
metadata commands use it. The columns used for reads and writes stay
nullable.

### Release note

Show the declared nullability of Iceberg columns in `DESC`, `SHOW CREATE
TABLE`, `SHOW COLUMNS` and `information_schema.columns`.

### Check List (For Author)

- Test
    - [x] Regression test
    - [x] Unit Test
- Behavior changed:
    - [x] Yes. Required Iceberg columns are now shown as `NOT NULL`.
- Does this need documentation?
    - [x] No.

…OW FULL COLUMNS (#69351)

### What problem does this PR solve?

Issue Number: None

Related PR: #67856, #68640

Problem Summary:

**In short.** Tests only. #68640 shows required Iceberg columns as NOT
NULL on master, as #67856 does on branch-4.1 and branch-4.2. The two
implementations behave the same (master also covers the HTTP schema APIs
and the MySQL protocol column metadata), but master's tests miss several
cases that the 4.x tests check. This PR adds them; it changes no
production code.

Original author: @hubgeter (the 4.x tests these cases come from)

**Background.** An Iceberg column declared `NOT NULL` (required) is read
by Doris as nullable, so scans and expressions keep their nullable
semantics, while DESC, SHOW CREATE TABLE, SHOW [FULL] COLUMNS and
information_schema show the declared constraint. On master,
`ConnectorColumn.sourceNullable` carries the declared nullability, and
`PluginDrivenExternalTable` projects a display view next to the read
schema in the same schema-cache entry.

**The problem, and what it cost.** These behaviors had no test on
master, so a regression in any of them would go unnoticed:

| Case | 4.x test | master before this PR |
|---|---|---|
| A table without any snapshot shows its required columns | yes | no
(the table was written before the first DESC) |
| Writing NULL into a required column is still rejected | yes | no |
| Relaxing a column creates no snapshot; information_schema, SHOW CREATE
TABLE and later writes follow it | yes | DESC only |
| SHOW FULL COLUMNS with the column comment, SHOW COLUMNS LIKE, `WHERE
Null = 'YES'`, information_schema filtered by IS_NULLABLE | yes | no |
| A hidden column keeps its declared nullability and is listed only in
the full view | yes | no (only the visible view) |

**How this PR fixes it.**

- `test_iceberg_show_nullable` (REST catalog): checks the empty table (0
snapshots) before the first write; adds SHOW FULL COLUMNS, SHOW COLUMNS
LIKE, `SHOW FULL COLUMNS ... WHERE Null = 'YES'` and information_schema
filtered by IS_NULLABLE; checks the column comment in SHOW CREATE TABLE;
writes NULL into `id` and `value` and expects "Column 'id' is declared
non-nullable but contains nulls" (the message the 4.x test expects too);
relaxes `value`, checks that the snapshot count stays 1 and that
information_schema and SHOW CREATE TABLE follow, then writes NULL into
`value` and still expects the rejection for `id`.
- `PluginDrivenExternalTableTest`: a new test with a hidden required
column checks that `getBaseSchemaForDisplay(full)` lists exactly the
columns of `getBaseSchema(full)`, in the same order, for both values of
`full`, and that the hidden column is NOT NULL only in the display view.
The setup of the existing display test moves into a helper both tests
use.

**Results.** Both pass locally. The blocks already in
`test_iceberg_show_nullable.out` are unchanged; 11 blocks are added. The
new unit test fails if the display view ignores `full`, or if a copied
display column loses its visibility.

Co-authored-by: daidai <[email protected]>