Fix force_parallel_mode to work as intended.

[avamingli]

The force_parallel_mode GUC is meant to facilitate testing of parallel query plans, but it currently allows non-parallel plans to win even when force_parallel_mode is enabled. This behavior makes it difficult to debug parallel plans as intended.

Correct the behavior for testing purposes.

Authored-by: Zhang Mingli [email protected]

Fixes #ISSUE_Number

What does this PR do?

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

Test Plan

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

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

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

Additional Context

CI Skip Instructions

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[avamingli]

This PR opened a whole can of worms. Just skimming through, there are a bunch of wrong results. Looks like it uncovered some hidden issues that’ll need to be checked one by one later.