Native clients that call PlaybackInfo, reuse the returned PlaySessionId on the
/Videos/{id}/stream?static=true request, but send the *item* id as mediaSourceId
(per the Jellyfin MediaSource.Id == Item.Id convention) rather than Silo's
fileID-based source id got a 400 "Media source is required" and could not
direct-play at all.
The PlaySessionId branch of resolvePlaybackRoute returned findMediaSource's nil
result directly, with no fallback -- unlike the FindByRoute and
createStaticPlaySession paths. Because resolution returned a nil error,
HandleVideoStream's Static fallback never fired either, so every direct-play
request 400'd.
Fall back to the session's primary source only when mediaSourceId is empty or
equals the route item id (the MediaSource.Id == Item.Id convention), matching
Jellyfin's StreamingHelpers. Any other unmatched id -- a stale/foreign id, or a
wrong version on a multi-version item -- stays rejected so it can't silently
stream the wrong file. Exact source-id matches are unchanged.
