fix: cover MUTATE_AFTER_SPLIT branches, guard empty/mismatched mutated statements

Add missing coverage for the MUTATE_AFTER_SPLIT=True branch, add type
hints to satisfy dev-standard lint, and guard against a SQL_QUERY_MUTATOR
that strips all statements or changes statement count when mutating
before the split.

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

tests/unit_tests/sql/execution/conftest.py

@@ -94,6 +94,10 @@ def mock_database() -> MagicMock:
     database.db_engine_spec.get_cancel_query_id = MagicMock(return_value=None)
     database.db_engine_spec.patch = MagicMock()
     database.db_engine_spec.fetch_data = MagicMock(return_value=[])
+    # Mirrors the real `Database.mutate_sql_based_on_config` default (no-op
+    # when no `SQL_QUERY_MUTATOR` is configured), so SQL parsed from its
+    # return value stays valid instead of an un-parseable `MagicMock`.
+    database.mutate_sql_based_on_config = MagicMock(side_effect=lambda sql, **kw: sql)
     return database