feat: expose SQL Server metadata comments

[debba]

Summary

• add optional comment fields to table and column metadata while preserving legacy payload compatibility
• read only MS_Description from sys.extended_properties with the correct object/column class, major_id, and minor_id scopes
• safely convert sql_variant descriptions to Unicode text and propagate column comments through batch, schema snapshot, and AI metadata paths
• cover apostrophes, Unicode, newlines, absent descriptions, exact property filtering, and host-model conformance

Testing

• just fmt
• just lint
• just test
• SQLSERVER_PLUGIN_BIN="$PWD/target/debug/sqlserver-plugin" SQLSERVER_TEST_HOST=127.0.0.1 SQLSERVER_TEST_PASSWORD='Str0ng!Passw0rd' cargo test --test live_db -- --test-threads=1
• SQL Server 2022 metadata smoke check for get_tables, get_columns, get_all_columns_batch, get_schema_snapshot, and get_ai_schema_context with described, undescribed, and unrelated-property objects
• git diff --check

Closes #23

[github-actions]

Version suggestion

Based on this PR's title (feat) and the prerelease:beta label:

Current	1.0.0-beta.1
Suggested next tag	v1.0.0-beta.2

This is informational only — no tag or release is created automatically yet.