orca: keep parallel semi joins from rewriting to dedup

[aviralgarg05]

Fixes #1593

What does this PR do?

This change keeps ORCA from rewriting semi-joins into inner-join-plus-deduplicate plans when parallel planning is enabled.

The issue behind orca_parallel was plan instability: in parallel mode, ORCA could choose rewrite paths that produced a dedup-based fallback shape instead of a direct semi-join. That made the expected plan unstable and, more importantly, pushed ORCA toward a plan form that is not desirable for parallel semi-join execution.

The fix is intentionally small and focused:

• In CXformLeftSemiJoin2InnerJoin
• In CXformLeftSemiJoin2InnerJoinUnderGb

both rewrites now decline to fire when gpdb::IsParallelModeOK() is true.

That preserves the existing rewrite behavior for non-parallel planning, while keeping parallel semi-joins on the direct semi-join path.

Type of Change

• Bug fix (non-breaking change)
• New feature (non-breaking change)
• Breaking change (fix or feature with breaking changes)
• Documentation update

Breaking Changes

None.

Test Plan

What I verified:

• Unit tests added/updated
• Integration tests added/updated
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Manual verification performed:

• Confirmed the touched ORCA xform objects compile cleanly
• Rebuilt the modified xform objects twice to make sure the change is deterministic

Notes:

• I attempted a fuller local build and regression run on macOS, but the tree currently hits unrelated baseline build failures with the local Apple toolchain before reaching the full regression path. The failures were outside this patch, so I did not modify unrelated files just to get the environment through.

Impact

Performance:

No broad performance change is intended.

This only affects ORCA’s choice space in parallel semi-join planning by preventing two rewrite paths that can lead to unstable dedup-based plans. In practice, this should make plan selection more predictable for the affected parallel queries.

User-facing changes:

Users should see more stable ORCA plans for affected parallel semi-join queries.

There is no SQL behavior change and no syntax change.

Dependencies:

None.

Checklist

• Followed contribution guide
• Added/updated documentation
• Reviewed code for security implications
• Requested review from cloudberry committers

Additional Context

This patch is intentionally narrow.

I avoided broader planner changes and did not touch regression files without a verified regenerated answer file from a clean local run. The code change is limited to the two ORCA xforms responsible for the semi-join-to-dedup rewrite path.

CI Skip Instructions

No CI skip requested.

[yjhjstz]

Disabling them removes a plan candidate that may have been the cost-optimal choice even for some serial queries in a parallel-enabled session, we need to find root cause .