Fix a bug which could corrupt auth chains (#18746)

[Complement tests](https://github.com/matrix-org/complement/pull/790)

Basically, when we use `/state_ids` in conjunction with `/event`
requests, the `/event` request can fail causing a partial state to be
returned. When we persist the state, we process dependent events first.
If we fail to process a dependent event due to missing the `auth_event`,
we didn't update the in-memory event map. This meant that we could
incorrectly persist events that depended on dropped events.

Discovered via Chaos testing.

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changelog.d/18746.bugfix

@@ -0,0 +1 @@
+Fix a bug which could corrupt auth chains making it impossible to perform state resolution.

synapse/handlers/federation_event.py

@@ -1728,6 +1728,9 @@ class FederationEventHandler:
                             event,
                             auth_event_id,
                         )
+                        # Drop the event from the auth_map too, else we may incorrectly persist
+                        # events which depend on this dropped event.
+                        auth_map.pop(event.event_id, None)
                         return
                     auth.append(ae)