Rich text editor sends HTML in plaintext body when mentioning users

### Steps to reproduce

1. Enable devtools and the rich text editor
2. Mention someone in a chat
3. Check the Matrix event that was sent

### Outcome

#### What did you expect?

An event that has a plaintext and a formatted body. That's how it looks for non-RTE:

```
"content": {
"msgtype": "m.text",
"body": "tadzik: hi",
"format": "org.matrix.custom.html",
"formatted_body": "tadzik: hi",
"m.mentions": {
"user_ids": [
"@irc_tadzik:home.tadzik.net"
]
}
},
```

#### What happened instead?

The event sent contains HTML in both `body` and `formatted_body`:

```
{
"msgtype": "m.text",
"body": "tadzik hi",
"format": "org.matrix.custom.html",
"formatted_body": "tadzik hi"
}
```

### Operating system

Debian Linux

### Browser information

Firefox 128 ESR

### URL for webapp

app.element.io

### Application version

Element version: 1.11.86 Crypto version: Rust SDK 0.7.2 (517d99b), Vodozemac 0.7.0

### Homeserver

Synapse v1.114.0

### Will you send logs?

No