fix(lint): add @file:OptIn(UnstableApi::class) to MediaSessionPool

Property-level @OptIn doesn't propagate through the lazy{} delegate body,
so lint flags the DataSourceBitmapLoader.Builder chain (lines 87-90) with
UnsafeOptInUsageError. File-level annotation is a one-line fix that lets
:amethyst:lintPlayDebug pass without changing runtime semantics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/playerPool/MediaSessionPool.kt

@@ -18,6 +18,8 @@
  * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
+@file:OptIn(UnstableApi::class)
+
 package com.vitorpamplona.amethyst.service.playback.playerPool
 
 import android.app.PendingIntent