to 3.0: fix: force CTAS follow-up insert to run on one CN

[gouhongshen]

What type of PR is this?

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

Which issue(s) this PR fixes:

issue ##23744

What this PR does / why we need it:

For CTAS, table creation and data population now happen in the same transaction.
the newly created target table (including hidden auto-increment/fake-pk metadata), causing pre-insert auto-increment
lookup to fail as no such table.

Fix

Before executing the CTAS follow-up internal SQL (createAsSelectSql), mark the current transaction as having DDL:

• pkg/sql/compile/ddl.go (normal CTAS path)
• pkg/sql/compile/ddl.go (temp table CTAS path)

This makes the internal SQL compile path choose one-CN execution, so the follow-up insert runs on the coordinator CN
and can access uncommitted metadata in the same transaction.

Why This Is Safe

• Scope is limited to CTAS internal follow-up SQL only.
• No user-visible SQL semantics are changed.
• This is a correctness-first mitigation; only CTAS internal insert distribution is constrained.

Impact

• Fixes CTAS failures in multi-CN for same-transaction create+insert scenarios.
• Potentially reduces parallelism for CTAS follow-up insert (expected and acceptable for correctness).