test: add regression test for RLS bypass via VALUES clause

[google-labs-jules]

The reported vulnerability allowed users to bypass Row Level Security (RLS) by embedding subqueries inside VALUES clauses in custom SQL metrics. This occurred because the table extraction logic failed to recurse into VALUES clauses, causing RLS predicates to be omitted for those tables.

This PR:

1. Confirms the vulnerability is fixed in the current codebase.
2. Identifies that the fix was achieved by switching the table extraction logic in superset/sql/parse.py to use sqlglot.optimizer.scope.traverse_scope (which correctly handles VALUES clauses) instead of a simpler AST traversal.
3. Adds a regression test test_extract_tables_nested_in_values to tests/unit_tests/sql/parse_tests.py to ensure this specific pattern is always detected.

The fix relies on sqlglot's scope traversal capabilities. The test confirms that with the current sqlglot version (pinned >= 27.15.2), the sales table is correctly extracted from the malicious payload.

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PR created automatically by Jules for task 1427205083747796665 started by @sha174n

[google-labs-jules]

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