The prior fix derived is_split purely from run_multiple_statements_as_one, so
for the common case (engines that execute statements individually) with the
default MUTATE_AFTER_SPLIT=False, is_split was always True and the mutator
never fired at all -- a regression from the pre-fix behavior where it always
ran. Mutate the whole, un-split query once before splitting into per-statement
blocks when MUTATE_AFTER_SPLIT is False, in both the sync (sql_lab.py) and
async (celery_task.py) SQL Lab paths, and add regression tests covering both
flag values across both engine modes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
