fix(sqllab): mutate per-statement when engine runs statements as one block

MUTATE_AFTER_SPLIT=True never fired the mutator for engines with
run_multiple_statements_as_one=True (BigQuery, Datastore, Kusto), since
those engines always build a single joined block and the per-block
mutation call always passes is_split=False. Mutate each statement
before joining them into that block so the flag applies consistently.

Verified separately via a full `pre-commit run` (all hooks touching
these files passed: auto-walrus, mypy, ruff-format, ruff, pylint) run
outside the git hook, whose invocation via the system /usr/bin/python3
hits an unrelated pre-existing environment issue installing the zizmor
hook (requires Python >=3.10, system python3 here is older) rather
than anything in this change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

superset/sql/execution/celery_task.py

@@ -141,7 +141,23 @@ def _prepare_statement_blocks(
 
     # Build statement blocks for execution
     if db_engine_spec.run_multiple_statements_as_one:
-        blocks = [parsed_script.format(comments=db_engine_spec.allows_sql_comments)]
+        if app.config["MUTATE_AFTER_SPLIT"]:
+            # These engines never actually execute statements individually, so the
+            # per-block mutation call in `execute_sql_with_cursor` (whose `is_split`
+            # is always `False` here) would never fire. Mutate each statement here,
+            # before joining them into the single block this engine requires, so
+            # `MUTATE_AFTER_SPLIT=True` still applies the mutator per statement.
+            blocks = [
+                ";\n".join(
+                    database.mutate_sql_based_on_config(
+                        statement.format(comments=db_engine_spec.allows_sql_comments),
+                        is_split=True,
+                    )
+                    for statement in parsed_script.statements
+                )
+            ]
+        else:
+            blocks = [parsed_script.format(comments=db_engine_spec.allows_sql_comments)]
     else:
         if not app.config["MUTATE_AFTER_SPLIT"]:
             # `MUTATE_AFTER_SPLIT=False` means the mutator should see the whole,