Deadline callback migration 0094 shallow-encodes nested callback kwargs → scheduler CrashLoopBackOff (KeyError('__var'))

### Apache Airflow version 3.2.0+ (root cause is migration `0094`, `airflow_version = "3.2.0"`; buggy code is still present on `main`) ### What happened On a deployment using **Deadline Alerts** with an `AsyncCallback` whose `kwargs` contain a **nested dict**, the **scheduler enters CrashLoopBackOff**. Every scheduler loop crashes while deserializing an associated `callback` row in the deadline-processing query: ``` File ".../airflow/jobs/scheduler_job_runner.py", line 1808, in _run_scheduler_loop for deadline in session.scalars( ... selectinload(Deadline.callback) ... ) File ".../airflow/utils/sqlalchemy.py", line 221, in process_result_value return BaseSerialization.deserialize(value) File ".../airflow/serialization/serialized_objects.py", line 632, in deserialize return {k: cls.deserialize(v) for k, v in var.items()} File ".../airflow/serialization/serialized_objects.py", line 629, in deserialize var = encoded_var[Encoding.VAR] KeyError: <Encoding.VAR: '__var'> ``` The crash happens before the scheduler heartbeats, so it presents as failing liveness probes / restarts with no OOM and no other logged exception — easy to misdiagnose as a probe or DB problem. ### Root cause Migration **`0094_3_2_0_replace_deadline_inline_callback_with_fkey.py`** (revision `e812941398f4`) moves the old inline deadline callback into the `callback` table. `callback.data` is an `ExtendedJSON` column, whose read path runs `BaseSerialization.deserialize` (`utils/sqlalchemy.py`), which requires **every nested dict to be wrapped** as `{"__type": "dict", "__var": {...}}`. But the migration hand-builds `callback.data` and only wraps the **top level**, embedding the old callback's `kwargs` **raw**: ```sql -- _upgrade_postgresql(): raw nested kwargs pulled from the old inline (SDK-serde) callback COALESCE(NULLIF(d.callback::jsonb->'__data__'->'kwargs', 'null'::jsonb), '{}'::jsonb) AS cb_kwargs ... INSERT INTO callback (... data ...) SELECT ... json_build_object( '__var', …