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/ui/playback/PlaybackViewModel.kt

@@ -192,14 +192,18 @@ class PlaybackViewModel
 
         val currentUserDto = serverRepository.currentUserDtoFlow
 
+        // Exposed for testing
+        val initJob: Job
+
         init {
-            viewModelScope.launchIO {
-                addCloseable {
-                    screensaverService.keepScreenOn(false)
-                    disconnectPlayer()
+            initJob =
+                viewModelScope.launchIO {
+                    addCloseable {
+                        screensaverService.keepScreenOn(false)
+                        disconnectPlayer()
+                    }
+                    init()
                 }
-                init()
-            }
         }
 
         private fun disconnectPlayer() {
@@ -269,10 +273,7 @@ class PlaybackViewModel
                     player,
                     preferences.appPreferences.playbackPreferences,
                 )
-            mediaSession =
-                MediaSession
-                    .Builder(context, sessionPlayer)
-                    .build()
+            mediaSession = playerFactory.createMediaSession(sessionPlayer)
         }
 
         /**
@@ -1054,7 +1055,8 @@ class PlaybackViewModel
         }
 
         // Variables for tracking segment state
-        private var segmentJob: Job? = null
+        // Exposed for testing
+        internal var segmentJob: Job? = null
         private val autoSkippedSegments = mutableSetOf<UUID>()
         private val outroShownSegments = mutableSetOf<UUID>()
 
@@ -1269,7 +1271,7 @@ class PlaybackViewModel
                                 }
                             }
                     } else {
-                        Timber.w("Attempting to play next, but there are no more items")
+                        Timber.w("Attempting to play previous, but there is none")
                         return@launchDefault
                     }
                 }