feat(media_session): map Android PlaybackState 6 to buffering (t55)

The media session parser only understood states 2 (paused) and 3
(playing), so a session refilling its buffer (Android state 6, which the
Shield's own ExoPlayer/MediaSession sessions report exactly like stock
Android) read as "idle" or vanished into the foreground tier.

MediaSessionInfo gains an independent `buffering` flag (playing/paused/
buffering are now three flags off one integer), the session sort puts
buffering right behind playing — it is mid-playback, so it outranks a
paused session when the ladder picks one — and the running-apps
enrichment names a buffering live/last-known session "buffering" instead
of falling into "idle".

The MediaPlayerState stub gains BUFFERING and STANDBY so an in-suite
reference to a member real HA does not have fails here (real HA 2026.10
exposes off/on/idle/playing/paused/standby/buffering).

tests/entity_stublib.py

@@ -79,6 +79,8 @@ class MediaPlayerState(enum.StrEnum):
     IDLE = "idle"
     PLAYING = "playing"
     PAUSED = "paused"
+    STANDBY = "standby"
+    BUFFERING = "buffering"
 
 
 class MediaPlayerEntityFeature(enum.IntFlag):