room version 12 error in unit test `tests.storage.databases.main.test_room.RoomBackgroundUpdateStoreTestCase.test_populate_stats_broken_rooms`

In working on bring up the unit tests for room version 12 compatibility, I hit a bit of an extremely niche error situation. This particular test fails with a `KeyError` on a missing `room_id` in creation events for v12 rooms. This appears to be because creation events for v12 rooms do not have a `room_id` in their JSON. This is to be expected due to [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291).

The test itself seems fine. The point of it is to "break" a room by removing the value in the `room_version` column in `rooms` before running. The "bug" appears to show inside the depths of the event fetching code. It is only hit when the `rooms` table is missing it's `room_version` value. EDIT: It also seems to happen when the room is an [unknown version](https://github.com/element-hq/synapse/blob/ace8447037a2d206fc1385b2d4969e6fb7445aa0/synapse/storage/databases/main/events_worker.py#L1493)

There was an ancient issue about [out-of-band invites](https://github.com/matrix-org/synapse/issues/6983) that led to introducing some simple guessing about what a room version should be based on the format version of an event. This conditional code is what is being hit with this test. But when trying to the raise a `InvalidEventError`, the `room_id` is missing so a `KeyError` is raised instead.

Traceback of exception

```
Traceback (most recent call last):
File "/home/jason/dev/famedly/synapse-upstreaming/tests/storage/databases/main/test_room.py", line 187, in test_populate_stats_broken_rooms
self.run_background_updates("populate_stats_process_rooms")
File "/home/jason/dev/famedly/synapse-upstreaming/tests/storage/databases/main/test_room.py", line 68, in run_background_updates
self.wait_for_background_updates()
File "/home/jason/dev/famedly/synapse-upstreaming/tests/unittest.py", line 495, in wait_for_background_updates
self.get_success(
File "/home/jason/dev/famedly/synapse-upstreaming/tests/unittest.py", line 742, in get_success
return self.successResultOf(deferred)
File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/trial/_synctest.py", line 732, in successResultOf
self.fail(
twisted.trial.unittest.FailTest: Success result expected on , found failure result instead:
Traceback (most recent call last):
File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/internet/defer.py", line 1853, in _inlineCallbacks
result = context.run(
File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/python/failure.py", line 467, in throwExceptionIntoGenerator
return g.throw(self.value.with_traceback(self.tb))
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/background_updates.py", line 577, in do_next_background_update
await self._do_background_update(desired_duration_ms)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/background_updates.py", line 620, in _do_background_update
items_updated = await update_handler(progress, batch_size)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/stats.py", line 221, in _populate_stats_process_rooms
await self._calculate_and_set_initial_state_for_room(room_id)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/stats.py", line 577, in _calculate_and_set_initial_state_for_room
state_event_map = await self.get_events(event_ids, get_prev_content=False) # type: ignore[attr-defined]
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/logging/opentracing.py", line 949, in _wrapper
return await func(*args, **kwargs)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/events_worker.py", line 624, in get_events
events = await self.get_events_as_list(
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/logging/opentracing.py", line 949, in _wrapper
return await func(*args, **kwargs)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/logging/opentracing.py", line 949, in _wrapper
return await func(*args, **kwargs)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/events_worker.py", line 680, in get_events_as_list
event_entry_map = await self.get_unredacted_events_from_cache_or_db(
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/logging/opentracing.py", line 949, in _wrapper
return await func(*args, **kwargs)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/events_worker.py", line 929, in get_unredacted_events_from_cache_or_db
missing_events: dict[str, EventCacheEntry] = await delay_cancellation(
File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/internet/defer.py", line 1187, in __iter__
yield self
File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/internet/defer.py", line 1857, in _inlineCallbacks
result = context.run(gen.send, result)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/events_worker.py", line 915, in get_missing_events_from_cache_or_db
raise e
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/events_worker.py", line 908, in get_missing_events_from_cache_or_db
db_missing_events = await self._get_events_from_db(
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/logging/opentracing.py", line 949, in _wrapper
return await func(*args, **kwargs)
File "/home/jason/dev/famedly/synapse-upstreaming/synapse/storage/databases/main/events_worker.py", line 1527, in _get_events_from_db
d["room_id"], event_id, original_ev.event_id
builtins.KeyError: 'room_id'
```

The chances of having this particular situation in a room version 12 room seem extraordinarily low, if not impossible so I would not say this is a high priority issue. That said I see multiple ways of remedying this test "failure"
* change the test to break in a different way
* adjust the event fetching code to accommodate events with no `room_id` in their raw JSON
* Begin catching a `KeyError` in addition to the `InvalidEventError` [here](https://github.com/element-hq/synapse/blob/1b0622fa99f4bf6aa235d0cee7f442f630ca6c51/synapse/storage/databases/main/stats.py#L576).
* Change the condition [here](https://github.com/element-hq/synapse/blob/1b0622fa99f4bf6aa235d0cee7f442f630ca6c51/synapse/storage/databases/main/events_worker.py#L1460) to include filtering out creation events. This has a side effect of needing to bring the section [below](https://github.com/element-hq/synapse/blob/1b0622fa99f4bf6aa235d0cee7f442f630ca6c51/synapse/storage/databases/main/events_worker.py#L1478-L1486) up-to-date to handle newer room versions or another error will be hit when converting the raw JSON into its `FrozenEvent*`. This then produces a different `event_id` to what is expected and tries to raise a `DatabaseCorruptionError` which fails with another `KeyError` instead because `room_id` is still missing from the raw JSON.
* Change [`EventRow`](https://github.com/element-hq/synapse/blob/1b0622fa99f4bf6aa235d0cee7f442f630ca6c51/synapse/storage/databases/main/events_worker.py#L158)(and it's associated SQL) to retrieve the `room_id` as a separate data point, there by avoiding the `KeyError` issue altogether.

I vote for the last one