"Key set not found" for user in encrypted video room

matrix-js-sdk: latest dev branch
matrix-react-sdk: latest dev branch
element-call: latest dev branch
LiveKit: 1.6.1

### Steps to reproduce

I am getting the following error in `encrypted` video rooms:
`livekit-client.e2ee.worker.mjs:1113 TypeError: key set not found for @john:domain.com:GPPDFBOMUK at index 0`

(It seems that) the key is not set here ...
https://github.com/element-hq/element-call/blob/0f38445fdd2254783a30898e5398a504a9906fbc/src/livekit/useLiveKit.ts#L79

### Outcome

The consequence is that users can join a video room but video and voice do not work in group calls.

All group calls work fine in a video room if the encryption is disabled for that room.

### Operating system

All

### Browser information

_No response_

### URL for webapp

_No response_

### Will you send logs?

No

Start with src/livekit/useLiveKit.ts at the referenced line and reproduce the reported failure in an encrypted group video room using the listed development branches and LiveKit 1.6.1. Trace why the user’s key set is unavailable, then verify that audio and video work in encrypted group calls without the reported worker error.

Written by the indexing model from the issue text.