Fix WHERE clause subquery extraction in parse_table_names()

[danielbodart]

Summary

• Fixes parse_table_names() missing tables referenced in WHERE clause subqueries
• Adds expression tree walking for all expression contexts (WHERE, SELECT list, HAVING, QUALIFY, GROUP BY, ORDER BY, JOIN conditions)
• Tables inside expression subqueries get subquery context

Example

-- Before:
SELECT parse_table_names('SELECT * FROM a WHERE x IN (SELECT y FROM b)');
-- Returns: [a]

-- After:
SELECT parse_table_names('SELECT * FROM a WHERE x IN (SELECT y FROM b)');
-- Returns: [a, b]

Test plan

• Added test cases for WHERE clause subqueries
• Added test cases for nested subqueries
• Added test cases for SELECT list, HAVING, JOIN condition, ORDER BY subqueries
• All existing tests pass

🤖 Generated with Claude Code

[danielbodart]

Closing - this fix was already implemented in PR #11. Thanks!