fix(plan): reject invalid ALTER COLUMN defaults

[ck89119]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #26844

What this PR does / why we need it:

Reject ALTER TABLE ... ALTER COLUMN ... SET DEFAULT for auto-increment and generated columns before mutating the copied table definition. This prevents successful no-op DDL when the requested default cannot be persisted.

Add planner unit tests for auto-increment, stored generated, and virtual generated columns, plus distributed SQL coverage that verifies rejected operations leave the schema unchanged and valid defaults still work.

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[aunjgr]

Reviewed exact head 2f4b955 after the successful CI rollup. The SET DEFAULT guards correctly reject auto-increment and generated columns before mutating the copied definition, with planner and BVT coverage.

[XuPeng-SH]

Deep review — approve

Reviewed exact head 2f4b955b0b26cd109cf0f9237fcd27df11f7c0f2 against merge-base 8be242b25bf9a44d73a3b1cc1db75fd0264a293c.

No blocking issue found.

The new guards reject SET DEFAULT for auto-increment and both stored and virtual generated columns before assigning to CopyTableDef, so planner failure cannot publish a partially changed schema. The normal-column path and DROP DEFAULT behavior remain unchanged. The test matrix includes the three rejected metadata states, a normal-column control, copied-definition immutability, and public SQL checks that verify post-error schema and insert behavior.

Correctness, compatibility, performance, and unhappy-path review are clean: the change adds only constant-time checks on a DDL planning path; the existing temporary AST remains released by its defer; no new waits, shared state, resource ownership, or unbounded growth are introduced. The branch is one unrelated commit behind current main, and the merge is clean.

Validation on the exact head:

• focused planner regressions passed;
• full ./pkg/sql/plan tests passed under the controlled CGo environment;
• planner build and vet passed;
• CI executed alter_table_set_default_restrictions.sql, and the required CI rollup is green.