Unit tests for PlaybackViewModel & several services (#1592)

## Description
This PR contains only tests, there are no user facing changes.

Adds tests for playback covering:
- Playing next, previous, or within the queue
- Media segments (intro/credit)
- Playing intros before media

Adds basic unit tests for track selection. This uses a new "test track"
builder which can model various files and build them as represented
(differently) by the server, ExoPlayer, and MPV. In future, this will
allow for testing of more complex files.

Also add unit tests for services covering things like:
- Fetching extras
- Deleting media
- Server/user management & auth

### Related issues
Will help with testing/verifying some issues like #1475, #1548 

### Testing
Ran the unit tests!

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/services/PlayerFactory.kt

@@ -24,6 +24,7 @@ import androidx.media3.exoplayer.trackselection.DefaultTrackSelector
 import androidx.media3.exoplayer.video.MediaCodecVideoRenderer
 import androidx.media3.exoplayer.video.VideoRendererEventListener
 import androidx.media3.extractor.DefaultExtractorsFactory
+import androidx.media3.session.MediaSession
 import com.github.damontecres.wholphin.preferences.AssPlaybackMode
 import com.github.damontecres.wholphin.preferences.MediaExtensionStatus
 import com.github.damontecres.wholphin.preferences.PlaybackPreferences
@@ -211,6 +212,11 @@ class PlayerFactory
                         ),
                 )
             }
+
+        fun createMediaSession(player: Player) =
+            MediaSession
+                .Builder(context, player)
+                .build()
     }
 
 val Player.isReleased: Boolean