duckdb_pglake: harden parallel partition scans

[sfc-gh-dachristensen]

Summary

Stacks on #653 to preserve PostgreSQL behavior while parallelizing scans of partitioned and inheritance tables.

• Refresh member discovery during scan initialization in the scan transaction, and use ONLY when scanning members directly.
• Query the parent when direct member reads could change grant or row-level security behavior, or when a pushed filter can benefit from partition pruning.
• If worker connections cannot share an exported snapshot, run one parent query so the scan does not split across connections with different member snapshots.
• Include pg_inherits membership in catalog staleness detection and reduce repeated partition metadata lookups.

Regression coverage

• LIMIT/OFFSET across partition tasks, including distinct-row assertions.
• Parent-only SELECT grants, parent row-level security, and filters on a partition key.
• Reset pg_pages_per_task after tests that change it.

Validation

• make release -j16: passed.
• test_postgres_scanner.py: 37 passed, 12 warnings (Pydantic deprecations).
• Focused LIMIT/OFFSET test: 1 passed.

🤖 Generated with Claude Code

[sfc-gh-dachristensen]

I rebased this onto the new #653 head (f42e965), which no longer has the inheritance-parent split (follow-up in #735). Changes from the previous version:

• Splitting by member is limited to partitioned tables again (relkind = 'p'). IsScannedByMember and the has_subclass column are gone, and the member queries go back to UNION ALL.
• Member COPYs no longer add ONLY. Members are leaf partitions now, and those can't have inheritance children.
• I removed the inheritance-parent fixtures and test_inheritance_parent_*. The LIMIT/OFFSET, grant, RLS and partition-key tests are unchanged.

Local checks: the regenerated patch set applies cleanly to the pinned duckdb-postgres, and the changed scanner sources compile. I haven't run the pytest suite locally; CI will cover it.

🤖 Generated with Claude Code