Generic hooks can still send very large events

Nobody has claimed this yet.

```
INFO 22:19:05:561 [GenericHookConnection] onGenericHook !fibble:element.io
ERROR 22:19:05:605 [MatrixHttpClient] (REQ-12802) { errcode: 'M_TOO_LARGE', error: 'event too large' }
WARN 22:19:05:606 [Bridge] Failed to handle generic webhook Error: Failed to send Matrix message
at MessageSenderClient.sendMatrixMessage (/bin/matrix-hookshot/MatrixSender.js:91:15)
at runMicrotasks ()
at processTicksAndRejections (node:internal/process/task_queues:96:5)
at async GenericHookConnection.onGenericHook (/bin/matrix-hookshot/Connections/GenericHook.js:311:9)
at async /bin/matrix-hookshot/Bridge.js:369:25
at async Promise.all (index 0)
at async LocalMQ. (/bin/matrix-hookshot/Bridge.js:350:13)
```

Start at Connections/GenericHook.js:311 and /bin/matrix-hookshot/MatrixSender.js:91, using the reported M_TOO_LARGE failure as the reproduction point. Trace how a generic hook event is built and sent, then establish the intended handling for oversized events and add coverage showing that this failure is handled correctly.

Written by the indexing model from the issue text.