C2S API: Reply includes automatic mention & manual mention

### Quick summary

WordPress automatically adds an `@mention` when replying to someone.
I'm not sure an ActivityPub API client should expect the server to automatically add the mention on a reply.

The second mention does not appear on the wordpress site, but does appear in the activitypub representation, so it is federated as such

### Steps to reproduce

Using an ActivityPub API client
1. Post a status
2. Have someone reply
3. Reply to the reply, adding a `@mention`
4. See double mention via browser.pub or on remote post being replied to.

See:
- https://browser.pub/https://mediaformat.org/?c=2355
- https://mastodon.libre-entreprise.com/@nuages@mediaformat.org/117080612985393797

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Moderate

### What other impact(s) does this issue have?

No revenue impact

### If a workaround is available, please outline it here.

_No response_