Rich text editor does not add m.mentions in events

### Steps to reproduce

1. Enabling Labs "Rich Text Editor"
2. Send a chat message with mentioning someone

### Outcome

#### What did you expect?

m.mentions should be existing in room event for the message sent.

#### What happened instead?

m.mentions is missing in room event for the message sent.

### Operating system

Windows 11 Pro 24h2

### Application version

Element version: 1.11.83 Crypto version: Rust SDK 0.7.2 (7a21514), Vodozemac 0.7.0

### How did you install the app?

https://packages.riot.im/desktop/install/win32/x64/Element%20Setup.exe

### Homeserver

Synapse 1.116.0

### Will you send logs?

No

Reproduce the issue with Labs “Rich Text Editor” enabled by sending a chat message that mentions someone, then inspect the generated room event and the rich-text message path. Done means the sent event contains the expected m.mentions field; no specific source file or test is named in the issue.

Written by the indexing model from the issue text.