Potential race when knocking to join a call over federation

### Steps to reproduce

(N.B. I suspect this is more of a Synapse problem, but reporting at application level.

There are two users here. Alice is on Server A, and Bob is on Server B.

1. Alice creates a new call, and sends the link along to Bob
2. Bob requests to join the call (knocks)
3. Alice accepts and invites Bo,b
4. Bob sees an error about not being able to join
5. (workaround) Bob is able to refresh and immediately join the call.

### Outcome

#### What did you expect?

Upon being inviited, I should be joined to the call.

#### What happened instead?


I suspect this is invites racing;

### Operating system

Arch Linux

### Browser information

Firefox

### URL for webapp

_No response_

### Will you send logs?

No

No files, tests, or code entry points are named. Start by reproducing the federated call flow with Alice on Server A and Bob on Server B, then trace the knock, acceptance, invite, and join events for ordering or timing problems; done means Bob joins immediately after acceptance without needing a refresh.

Written by the indexing model from the issue text.