Element Web fills in mentions information for code blocks & spoiler tags

### Steps to reproduce

Sending a code-block or a spoiler tag with a pill of another user should not add that user to the mentions property (per MSC3952), it is a bit weird to reproduce, but:

1. Enable intentional mentions in the labs flags
1. Create a code block (triple ticks)
2. Mention a user (`@clokep`)
3. Close the code block (triple ticks)
4. Send the message
5. View the resulting message source

Alternately for spoiler tags:

1. Enable intentional mentions in the labs flags
1. Type `/spoiler @clokep`
4. Send the message
5. View the resulting message source

### Outcome

#### What did you expect?

I did not expect the mentioned user to be placed in the `org.matrix.msc3952.mentions` property and the event to be rendered as normal code.

#### What happened instead?

1. The user is placed into the mentions property.
2. The *message* itself doesn't have a pill, however (there's no matrix.to link).

Message source for code block:

```json
{
"room_id": "!asdf,
"type": "m.room.message",
"content": {
"msgtype": "m.text",
"body": "Hello:\n\n```\nclokep \n```",
"format": "org.matrix.custom.html",
"formatted_body": "

Hello:

\n

[clokep](https://matrix.to/#/@clokep:matrix.org) \n

\n",
"org.matrix.msc3952.mentions": {
"user_ids": [
"@clokep:matrix.org"
]
}
}
}
```

Message source for spoiler:

```json
{
"room_id": "!asdf",
"type": "m.room.message",
"content": {
"msgtype": "m.text",
"format": "org.matrix.custom.html",
"body": "Hi @clokep:matrix.org",
"formatted_body": "Hi @clokep:matrix.org",
"org.matrix.msc3952.mentions": {
"user_ids": [
"@clokep:matrix.org"
]
}
}
}
```

### Operating system

All

### Browser information

N/A

### URL for webapp

develop.element.io

### Application version

Element version: 1.11.30 Olm version: 3.2.12

### Homeserver

Ask me

### Will you send logs?

No

Reproduce the issue in develop.element.io with intentional mentions enabled, using both a triple-backtick code block and the /spoiler command, then inspect the resulting message source. Trace the mention extraction and formatted rendering paths for these inputs; done means users inside code blocks and spoiler tags are not added to org.matrix.msc3952.mentions and code remains rendered normally.

Written by the indexing model from the issue text.