Expose table and column comments in metadata responses

[debba]

Context

Core support is being added in TabularisDB/tabularis#764 for TabularisDB/tabularis#722. The runtime now accepts an optional comment: string | null in get_tables and get_columns results.

The built-in PostgreSQL driver is covered by the core PR, but the separately distributed PostgreSQL plugin needs parity. PostgreSQL stores table and column descriptions in pg_description.

Requested changes

• Update get_tables to return table descriptions as optional comment values.
• Update get_columns and row_to_table_column to return column descriptions as optional comment values.
• Join pg_description through the schema-qualified pg_class / pg_attribute objects and constrain classoid = 'pg_class'::regclass:
  • table: objsubid = 0
  • column: objsubid = pg_attribute.attnum
• Return null or omit the property when no comment exists.
• Add integration tests for comments containing apostrophes, Unicode, and newlines, plus objects without comments.
• When the currently stubbed bulk metadata methods are implemented, ensure they return the same column comment metadata.

Compatibility

This is additive: no manifest/capability change or minimum runtime bump is required. Older Tabularis runtimes ignore unknown JSON fields, while newer runtimes continue to accept payloads without comment.

References

• Runtime contract, built-in implementation, and UI: feat: expose table and column comments tabularis#764
• PostgreSQL object comments: https://www.postgresql.org/docs/current/catalog-pg-description.html
• PostgreSQL COMMENT: https://www.postgresql.org/docs/current/sql-comment.html