Expand pg_catalog and information_schema compatibility for GUI introspection

[MPCoreDeveloper]

Summary

Expand PostgreSQL metadata compatibility (pg_catalog, information_schema) to improve GUI introspection.

Why

GUI tools rely on catalog views to discover tables, indexes, constraints, triggers, and relationships.

Scope

• Implement/expand required catalog views and columns used by major clients.
• Ensure object discovery works for indexes, PK/FK, constraints, triggers.
• Improve metadata consistency for DDL round-trip scenarios.

Implementation Plan

1. Trace metadata queries emitted by target tools.
2. Prioritize missing views/columns by impact.
3. Implement metadata mapping layer and tests.
4. Validate with compatibility matrix scripts.

Acceptance Criteria

• External tools can introspect core schema objects without major fallback failures.
• Metadata query test suite added.
• Documentation updated with supported catalog coverage.

Dependencies

• Depends on compatibility matrix findings.