Historical state events from `state_events_at_start` in the MSC2716 `/batch_send` endpoint get pushed to an application service `/transactions`

[MadLittleMods]

As discovered by @tulir, when using the MSC2716 /batch_send endpoint, events specified in state_events_at_start will come down incremental /sync and the application services /transactions endpoints. Since this is historical state, it shouldn't be coming down the live pipe.

We already have a Complement tests to ensure the historical messages don't come down incremental sync, but we should also do the same for the state events.

Relevant code

• Where we create state events from the state_events_at_start: synapse/handlers/room_batch.py#L213-L249
• builder.internal_metadata.historical = True: synapse/handlers/message.py#L608
• If it's a historical event, then we mark it as backfilled = True: synapse/handlers/message.py#L1526-L1530

[MadLittleMods]

From my testing, the historical state does not come down /sync thanks to it being an outlier already. But we should make sure it's also historical in the internal metadata.

Need to dig into the application service being pushed historical state events on /transactions by the homeserver which was the original report anyway.

I just assumed it would be a problem on incremental /sync if it was pushed on /transactions.