fix: keep field metadata when CASE WHEN false simplifies to NULL - #26951

Which issue does this PR close?

• Part of CASE expressions drop field metadata, silently stripping Arrow extension types and breaking metadata-aware UDFs #24860.

Depends on #26145. This PR makes the simplifier copy the CASE's field metadata onto the NULL it folds to. On main, a CASE's field has no metadata yet (#26145 fixes that), so this change has no effect there, and the new tests fail until #26145 merges. They pass with this commit applied on top of #26145.

Rationale for this change

CASE WHEN false THEN x END (including forms that constant-fold to false, such as WHEN 1 = 0) loses the Arrow extension type of x. Planning sees the CASE's field, but the simplifier then replaces the CASE with a bare NULL literal without metadata, so a function that checks its argument's extension type sees a different type after optimization than during planning.

With #26145 and #26158 applied, this is the only remaining place where the following query loses the extension type:

select arrow_metadata(
  case when 1 = 0 then with_metadata(name, 'ARROW:extension:name', 'a.type') end,
  'ARROW:extension:name')
from table_with_metadata;
-- a.type with this PR + #26145; NULL without this PR

EXPLAIN shows the CASE has already become Utf8(NULL) before physical planning, so the physical CASE fix in #26158 does not reach it.

What changes are included in this PR?

• CASE WHEN false THEN A END --> NULL (every WHEN is false and there is no ELSE) now builds the NULL from the CASE's own field: its type and its metadata. The merge policy stays in Expr::Case's to_field (fix: preserve CASE result field metadata #26145); the simplifier does not compute it separately.
• The other folds in the same rule are unchanged. CASE WHEN false THEN A ELSE B END --> B and CASE WHEN true THEN A ... END --> A already return the surviving branch, which carries its own metadata.

What is the testing strategy for this PR?

• metadata.slt: the query above, checking the extension type at execution after the fold.
• expr_simplifier.rs: simplify_expr_case_when_false_keeps_field_metadata, checking the folded literal carries the metadata of the column it replaced.
• The existing CASE simplifier tests still pass.
• Run locally: on main, only the two new tests fail (the literal has no metadata). With this commit on top of fix: preserve CASE result field metadata #26145, all datafusion-optimizer lib tests and metadata.slt pass.

Are there any user-facing changes?

A CASE that is folded to NULL keeps its field metadata.