Properly verify and error on power-level mismatch in /batch_send endpoint

[sumnerevans]

Closes #13216

Signed-off-by: Sumner Evans sumner@beeper.com

Outstanding Problem

In the test_batch_send_with_permission_msc2716_room_version test, I set the historical power level to 0 before calling the /batch_send endpoint: https://github.com/matrix-org/synapse/pull/13229/files#diff-7ec980d681db7426675aa46dfb312defa64ba0a6fa763f97eac107e9567c7e7fR438-R451

However, when the batch-send endpoint gets to the permission checks, it gets the power levels and historical is at 100, making the test fail.

Things I've done to debug include:

• Ran self.helper.get_state again after the send_state to ensure that the power level is correct. This returned the expected value for historical of 0.
• In get_named_levels, when the power levels event is retrieved, it has historical set to 100, which makes me think that maybe the auth_events being passed in are stale?

Clues I have:

• When the insertion event is created using self.event_creation_handler.create_event, it gets initial_state_event_ids passed in. This doesn't include the power levels event.

Asked for spec clarification here: https://github.com/matrix-org/matrix-spec-proposals/pull/2716/files#r924472086

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[sumnerevans]

Closing this due to the following reasons:

• it's not entirely clear what should be happening here according to the spec, and I don't want to keep re-implementing this every time the spec changes.
• Beeper is no longer going to be using Synapse's batch send endpoint as we are using an alternative homeserver implementation for bridges.

Anyone who is interested in implementing this functionality is welcome to use this as a starting point.