Fix PostgreSQL key metadata for readonly users

[DhruvShah-Dev]

Summary

• Replace PostgreSQL primary-key column detection with a pg_catalog query instead of information_schema.table_constraints.
• Reuse the catalog expression across single-table, batch-table, and view column loading.
• Extend coverage so the Postgres FK integration scenario also verifies primary-key detection on the referenced table.

Why

Issue #96 reports that readonly users can have access to pg_constraint while information_schema.table_constraints returns no rows. That caused key metadata to be missed even when the required catalog data was visible.

Validation

• pnpm.cmd typecheck
• pnpm.cmd lint
• pnpm.cmd build
• git diff --check
• pnpm.cmd exec gitnexus detect-changes reported low risk

[debba]

Thanks a lot for this contribution! The fix looks solid: the pg_catalog-based EXISTS expression is correlated on schema, table, and column, handles composite keys via unnest(conkey), and correctly skips dropped columns. I reproduced the original issue locally with a SELECT-only role — information_schema.table_constraints returns no rows for it while the new pg_constraint query correctly reports the primary key — so this genuinely fixes #96. Unit tests (102 passed) and the Postgres integration test both pass on the branch. Merging.