Rejected events not handled correctly during state resolution

[squahtx]

The spec seems to imply that events rejected due to failing auth at their state may be later accepted by state resolution: https://spec.matrix.org/v1.2/rooms/v9/#rejected-events

This might or might not also include events that pass auth at their state, but were rejected because they cite an auth event that didn't pass auth at its state.

Synapse handles this badly, as it does not distinguish between events rejected due to state checks and events rejected because they are malformed, etc. Once Synapse has rejected an event, it expects it to remain rejected and various things break if a rejected event later becomes accepted.