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.

SQL Server commonly stores object descriptions in the MS_Description extended property, exposed through sys.extended_properties.

Requested changes

• Add optional comment fields to the plugin's TableInfo and TableColumn response models.
• In get_tables, left join the MS_Description property with class = 1 and minor_id = 0 and map its value to TableInfo.comment.
• In get_columns, join the corresponding property using the table object_id and column column_id, mapping it to TableColumn.comment.
• Keep bulk metadata paths consistent, where implemented.
• Cast sql_variant values safely to text; return None / omit the property when absent.
• Add tests for descriptions containing apostrophes, Unicode, and newlines, plus objects without descriptions.

Only the conventional MS_Description property should be interpreted as the description; unrelated extended properties must be ignored.

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 and UI: feat: expose table and column comments tabularis#764
• SQL Server extended properties catalog: https://learn.microsoft.com/en-us/sql/relational-databases/system-catalog-views/extended-properties-catalog-views-sys-extended-properties
• sp_addextendedproperty: https://learn.microsoft.com/en-us/sql/relational-databases/system-stored-procedures/sp-addextendedproperty-transact-sql