Invites to unnamed rooms don't synthesise the right name.

### Steps to reproduce

1. Invite a 3rd person to a DM
2. Observe that the invite they receive is misnamed and just says it came from you, rather than being an invite to an existing DM.

### Outcome

#### What did you expect?

The server should hopefully send us enough info about the invite in the invite state to be able to synthesise a consistent room name. If it doesn't, it should.

#### What happened instead?

If Alice invites Charlie to her existing DM with Bob, Charlie's EI sees an invite from Alice to a room called Alice, not a room called Alice & Bob.

### Your phone model

_No response_

### Operating system version

_No response_

### Application version

1.8.13 tf

### Homeserver

_No response_

### Will you send logs?

No

Reproduce the iOS invite flow by inviting a third person to an existing direct message. Inspect how the invite state determines the room name, then verify that the invite consistently identifies the existing participants, such as “Alice & Bob,” rather than only the inviter. No source file or test is named in the issue.

Written by the indexing model from the issue text.