Do not add server mentions to replies written through the ActivityPub API

Follow-up from #3646 / #3680.

A reply written through the ActivityPub API arrives with the client's own mentions, and the reporter's position (https://github.com/Automattic/wordpress-activitypub/pull/3680#issuecomment-5454991704) is that committing to the API should mean the server adds no mention of its own to such a reply at all, and leaves mentions to the client and the user. I think that is right, and it is how Mastodon treats client content. #3680 does not do it; it removes the duplicate, which is the visible symptom, and leaves the reply context in place.

The reason it cannot be done in #3680 is that a client-authored comment is not distinguishable from a comment a logged-in user typed into the WordPress form. `Interactions::activity_to_comment()` writes `user_id` on the outbound branch and nothing else; `protocol` and `source_id` are only written for inbound remote comments. So there is nothing for `Transformer\Comment::get_content()` to check.

Proposed shape:

* Mark comments created through the outbox as client-authored, on the `$user_id` branch of `activity_to_comment()`. `source_id` set to the object id, the way inbound already does, plus a marker for the origin.
* In `Transformer\Comment::get_content()` (and `get_mentions()`), skip `extract_reply_context()` for a client-authored comment. The client sent the content; the server should not rewrite it.
* WordPress-native replies keep the reply context, since a comment form has no way to type a mention.

Existing comments have no marker, so the skip only applies to comments created after the change. The dedupe from #3680 stays as the safety net for those.

Start with Interactions::activity_to_comment() and trace the $user_id branch that creates comments from ActivityPub API replies. Then inspect Transformer\Comment::get_content() and get_mentions(), including extract_reply_context(), to see how the origin marker should affect processing. Done means client-authored replies retain only client-provided mentions, while WordPress-native replies keep their reply context and existing deduplication remains effective.

Written by the indexing model from the issue text.