fix(pg-catalog): restrict unqualified pg table rewrite

[discord9]

Summary

• Restrict PrependUnqualifiedPgTableName to known PG_CATALOG_TABLES instead of every unqualified pg_* name.
• Preserve unqualified pg_catalog relation support such as pg_namespace and pg_type.
• Add regression coverage so user tables like pg_compat_test are not rewritten to pg_catalog.

This keeps the PostgreSQL implicit pg_catalog search path compatibility while avoiding false positives for user-defined tables whose names start with pg_.

Related: GreptimeTeam/greptimedb#8359

Tests

• cargo test --manifest-path datafusion-pg-catalog/Cargo.toml test_prepend_unqualified_table_name
• cargo test --manifest-path datafusion-pg-catalog/Cargo.toml --lib

[sunng87]

Looks good!