Fix PostgreSQL 15 RLS predicate comparison context

[MelbourneDeveloper]

TLDR

Fix PostgreSQL 15 RLS migrations by comparing live and desired predicates in the same PostgreSQL relation context. Fixes #119.

What Was Added?

• Normalized PostgreSQL schema inspection for the migration CLI.
• A PostgreSQL 15.1 test collection reusing the shared PostgreSQL fixture.
• Nine CLI/integration regressions covering fresh creation, unchanged reapply, USING and WITH CHECK changes, WITH CHECK-only INSERT policies, qualified/quoted columns, functions, negation, correlated subquery shadowing, and pre-commit rollback.

What Was Changed or Deleted?

• Both initial PostgreSQL CLI inspection and inspection inside the pre-commit integrity callback normalize catalog policies through the same temporary-view mechanism already used for desired predicates.
• Shared test fixture initialization accepts an explicit image; existing suites retain PostgreSQL 16.
• Predicate qualification remains intact. Predicate drift detection, integrity verification, and transaction rollback remain enforced.

How Do The Automated Tests Prove It Works?

• Before the fix, six PostgreSQL 15 regression cases failed with the reported predicate-drift integrity error. The same minimal case passed on PostgreSQL 16, explaining the previous coverage gap.
• All nine PostgreSQL 15 cases now pass. The rollback test requires the exact integrity-rejection error and strict equivalence of the catalog policy after rollback, preventing an unrelated callback failure from satisfying the assertion.
• Installed public DataProviderMigrate 0.10.0-beta reproduced the exact issue. The rebuilt CLI succeeds with that same YAML and database, then reruns with zero operations.
• Full make ci passed after the latest edits against committed main plus these six files. Deslop is below the committed duplication ceiling; no new test clusters were reported.
• Actual Pi read-only security review found no blockers.

Breaking Changes

• None