sql: expand stars directly inside ROW expressions

[Alignyx]

Summary

Expand stars that are direct children of an explicit ROW(...) constructor into the enclosing row expression, matching PostgreSQL tuple semantics.

Fixes #26619

Root cause

The expression visitor normally replaces one expression with one expression. For ROW((pg_get_keywords()).*), the star expands to several columns, but the visitor cannot splice those columns into the parent tuple. The expanded value was therefore retained as a nested tuple-valued expression and later rendered as a single string-like field. PostgreSQL instead replaces the direct star with the expanded fields, producing one row with the expected columns.

The syntax distinction matters: an explicit ROW(...) constructor has different expansion semantics from shorthand tuple expressions. Applying the splice to every tuple would change existing behavior for shorthand tuples and regress the counterexample covered by the surrounding name-resolution logic.

Change

Handle tree.Tuple nodes before the ordinary child walk:

1. Restrict the new behavior to tuples marked as explicit rows.
2. Normalize direct variable names before checking for AllColumnsSelector or TupleStar children.
3. Expand each direct star with the existing expandStar implementation and append all resulting expressions to the enclosing row.
4. Continue walking non-star children normally.
5. Rebuild the tuple while preserving its explicit labels and original tuple metadata.

Nested stars and shorthand tuples continue through the existing path. The change is limited to direct-star expansion inside ROW(...), so it fixes the reported case without changing unrelated tuple construction.

Validation

• Reproduced SELECT ROW((pg_get_keywords()).*) and verified that the expanded fields form the enclosing row instead of a nested string-like value.
• Verified shorthand tuple behavior remains unchanged.
• Ran the affected optimizer/name-resolution module tests, native regression checks, held-out checks, and the full regression suite.

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