Add identifier field to PendingPlaylist in PlayOutcome

Updated `PendingPlaylist` to include a new `identifier` field, enabling more precise playlist identification. Adjusted `SearchResultHandler` and `MusicRoutes` to use and expose this field, ensuring integration across the system and improving API consistency.

src/main/java/me/bobert/netneutrality/http/server/MusicRoutes.java

@@ -643,6 +643,7 @@ public final class MusicRoutes {
                 body.put("status", "pending_playlist");
                 body.put("search_id", pendingPlaylist.searchId());
                 body.put("search_id_ttl_seconds", PendingSearchResults.ttlSeconds());
+                body.put("identifier", pendingPlaylist.identifier());
                 body.put("name", pendingPlaylist.name());
                 body.put("tracks", pendingPlaylist.tracks().stream().map(MusicRoutes::trackSummary).toList());
                 body.put("truncated", pendingPlaylist.truncated());

src/main/java/me/bobert/netneutrality/music/PlayOutcome.java

@@ -23,7 +23,7 @@ public sealed interface PlayOutcome {
     record PendingTrack(String searchId, Track track) implements PlayOutcome {
     }
 
-    record PendingPlaylist(String searchId, String name, List<Track> tracks, boolean truncated) implements PlayOutcome {
+    record PendingPlaylist(String searchId, String identifier, String name, List<Track> tracks, boolean truncated) implements PlayOutcome {
     }
 
     record PendingSearch(String searchId, List<Track> candidates) implements PlayOutcome {

src/main/java/me/bobert/netneutrality/music/SearchResultHandler.java

@@ -66,7 +66,7 @@ public class SearchResultHandler extends AbstractAudioLoadResultHandler {
         List<Track> candidates = truncated ? tracks.subList(0, maxPlaylistSize) : tracks;
 
         String searchId = PendingSearchResults.put(guild.getId(), candidates);
-        future.complete(new PlayOutcome.PendingPlaylist(searchId, result.getInfo().getName(), candidates, truncated));
+        future.complete(new PlayOutcome.PendingPlaylist(searchId, identifier, result.getInfo().getName(), candidates, truncated));
     }
 
     @Override