…n table (#69227)

### What problem does this PR solve?

Issue Number: None

Related PR: #66466, #68242

Problem Summary:

**In short.** Ports the primary-key Variant coverage of branch-4.1
#66466 to master: a Paimon deduplicate table whose second snapshot
updates two keys, and three blocks of `test_paimon_catalog_variant` that
read it through JNI. The rest of #66466 and the WAL fix #68242 are not
ported, because master handles those cases another way; see below.

**Background.** #66466 did three things on branch-4.x:

1. In BE, a generic gate in `TableReader::validate_file_mapping` rejects
a physical Variant read from any file format but Parquet. #68242 then
exempted WAL, which stores Doris blocks and supports Variant, because
the gate broke group commit WAL replay.
2. In FE, `PaimonScanNode` rejects a query that projects a Variant
column while Variant V2 is disabled.
3. In the tests, `run13.sql` gets `variant_primary_key_smoke`, and
`test_paimon_catalog_variant` reads it.

**The problem, and what it cost.** Every Variant table master's
`test_paimon_catalog_variant` reads is append-only, so the regression
does not check that a Paimon merge read keeps only the latest row of
each key while it materializes Variant values.

**How this PR fixes it.**

- `run13.sql` creates `variant_primary_key_smoke` as on branch-4.x: a
deduplicate primary-key table in Parquet, written in two snapshots. The
second updates ids 1 and 2 and inserts id 4; id 3 holds a root array.
- The JNI section of `test_paimon_catalog_variant` gets the three
branch-4.x blocks: the whole Variant value, Variant paths cast to
scalars, and a predicate on a Variant path. All three expect the latest
value of ids 1 and 2.

The other parts of #66466 and #68242 do not apply to master:

| Branch-4.x change | Master |
|---|---|
| FE rejects a Variant projection while Variant V2 is disabled | #66858
made Variant V2 the default and removed the switch, so there is nothing
to check |
| BE rejects a physical Variant read from any format but Parquet | Would
reject the native Variant reads of the ADBC reader (#68883). Master
keeps the Iceberg-specific gate in `IcebergTableReader`, admits Arrow
Variant reads in `FileScannerV2` only for ADBC, and plans a Paimon split
that projects a Variant natively only when every file is Parquet or an
ORC file whose schema predates the Variant column. Paimon 1.4.2 cannot
create an ORC table with a VARIANT column (`Unsupported type: VARIANT`)
|
| WAL is exempted from that gate | WAL is not gated on master |

**Results.** On a local cluster against the Iceberg docker environment,
Spark reads `variant_primary_key_smoke` as four rows, with the
second-snapshot values for ids 1 and 2. Doris returns the same rows in
the three new blocks, and every other result block of the suite is
unchanged.

Co-authored-by: zhangstar333 <[email protected]>