SQL Lab passed the default is_split=False to mutate_sql_based_on_config for
every block, so SQL_QUERY_MUTATOR fired only when MUTATE_AFTER_SPLIT=False and
never when True. Pass is_split based on whether the engine runs statements as
one block, matching the canonical usage in db_engine_specs and the new
execution engine, and thread the same flag through execute_sql_with_cursor so
the async (Celery) path is consistent. Adds unit tests for both the guard
semantics and the executor contract, plus a config note.
Closes#30169
Supersedes #34111
Co-authored-by: Lucas Wolkersdorfer <lucas.wolkersdorfer@rise-world.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
