allow for audio filters to be registered after initial room connection

[lukasIO]

Problem

Audio-filter plugins are authenticated in a one-time pass: when a room
connects, FfiRoom::connect calls on_load(url, token) for every plugin
registered at that moment. A plugin registered after the room has
connected is never initialized for it — its later create then fails
because the connection was never authenticated (surfacing downstream as
"failed to initialize the audio filter").

Because room.connect() is async, any registration that lands in that
window loses the race.

Fix

When a plugin is registered (LoadAudioFilterPlugin), also run its on_load
against every already-connected room, using each room's connect URL and
current token — so registration order no longer matters.

• New Room::token() accessor (livekit): current signalling token, needed to
  authenticate late-registered plugins.
• New FfiServer::list_rooms() (livekit-ffi): snapshot of connected rooms
  (refactored out of the existing dispose logic).
• on_load_audio_filter_plugin runs on_load per room on spawn_blocking
  (fire-and-forget), so a plugin whose on_load does network I/O can't stall
  the request thread. on_load is idempotent per URL, so this can't conflict
  with the connect-time pass. Registration happens before the loop so a
  concurrently-connecting room is still covered by the normal path.

Notes

• Uses the room's connect URL (not the resolved signalling URL) so the auth
  key matches what create later looks up.

[github-actions]

Changeset ✓

This PR includes a changeset covering all affected packages:

Package	Bump
livekit	patch
livekit-ffi	patch

[xianshijing-lk]

lgtm if you add the test.