Replace std::shared_ptr<File> by std::unique_ptr<File>

[gfphoenix78]

Shared smart pointer will introduce additional cost, and it also make the ownership complicated. Use unique pointer to make the ownership explicitly visible.

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

---