Notify Mastodon apps about received messages - #711

Fixes akirk/enable-mastodon-apps#320

A direct message received through a Mastodon app produces no notification, so the only way to notice one is to open the conversations view.

Changes

• Messages are only exposed to Enable Mastodon Apps through the conversation filters — they were never part of the notification query. They are now registered as a notification source through the mastodon_api_get_notifications_queries filter added in Query every notification source separately enable-mastodon-apps#331, which queries each source on its own. That separation matters here: the query this integration already contributes for cached posts carries a tax_query for the current user's mention tag, and messages don't carry that tag.
• On an Enable Mastodon Apps release without that filter this is inert, so it can go in independently.
• A test asserts that the messages are registered as their own source.

Testing instructions

• With Friends, ActivityPub and Enable Mastodon Apps active, receive a direct message from a friend and open the notifications in a Mastodon app: it now shows up as a mention.
• Exercised in a disposable WordPress with four things waiting for the user: a comment on their post, a DM in EMA's own DM post type, a cached public mention carrying the mention tag, and a message received through Friends.

=== this branch + EMA with the new filter ===
notifications: 4
  mention  post_type=friend_message      <- new
  mention  post_type=friend_post_cache
  mention  post_type=ema-dm-1
  mention  post_type=comment

=== this branch + EMA main (no new filter) ===
notifications: 1
  mention  post_type=friend_post_cache   <- unchanged from before

Changelog entry

• Automatically create a changelog entry from the details below.

https://claude.ai/code/session_01CUVxR8uyLTg2roNVWgp91y

Test this PR in WordPress Playground

You can test this pull request directly in WordPress Playground:

Launch WordPress Playground

This will install and activate the plugin with the changes from this PR.