Allow DESCRIBE/SHOW metadata statements in common.ai SQLToolset

[kaxil]

In read-only mode, the common.ai SQLToolset query tool only accepted SELECT-family statements (Select, Union, Intersect, Except). Read-only metadata statements such as DESCRIBE TABLE and SHOW TABLES / SHOW COLUMNS were rejected with SQLSafetyError. Agents frequently open with DESCRIBE to learn a table's columns, so a run that composed SELECTs directly succeeded while one that started with DESCRIBE failed outright, making agent runs nondeterministic.

The query and check_query tools now also accept read-only metadata statements (DESCRIBE/DESC and SHOW) when allow_writes=False (the default).

How it works

• validate_sql() gains an opt-in allow_read_only_metadata flag that widens the read-only allow-list with exp.Describe and exp.Show. Only SQLToolset sets it, so LLMSQLQueryOperator keeps its SELECT-family-only contract.
• SQLToolset passes the connection's dialect to the validator. SHOW only parses to a metadata statement on dialects that support it (Snowflake, MySQL); without a supporting dialect sqlglot falls back to a command statement that stays blocked. DESCRIBE parses to a metadata statement on every dialect.
• The data-modifying deep scan still runs and now also rejects DDL nodes (CREATE/DROP/ALTER/TRUNCATE), so writes wrapped behind DESCRIBE/EXPLAIN (e.g. EXPLAIN DELETE ..., DESCRIBE DROP TABLE ...) remain blocked.
• The SQLAlchemy-to-sqlglot dialect mapping is consolidated into a shared resolve_sqlglot_dialect() helper (reused by LLMSQLQueryOperator) that returns None for unknown dialects, so a misdetected dialect never breaks validation.

Usage

No API change. In the default read-only mode an agent can now run, for example, DESCRIBE TABLE my_table or (on databases that support it) SHOW COLUMNS FROM my_table.

Gotchas

• SHOW is only recognized on databases whose dialect sqlglot supports; elsewhere it stays rejected (those databases generally do not support SHOW anyway).
• Like SELECT, metadata statements are not scoped by allowed_tables (a documented visibility hint, not access control). Use database permissions to restrict access.