fix(music): convert SAF content:// downloads for gapless arming

setNext() appended raw content:// URIs that mpv cannot open, stalling
playback at the SD-card-download track boundary. Convert to fdclose://
like open(), track the armed fd, and reclaim it via a new closeContentFd
method when the entry is dropped unplayed (close only when provably
unconsumed — playlist-pos 0 before and after the remove; leak on doubt).
The playlist-pos pre-check also keeps the clear path from removing the
playing entry when mpv rolls into the armed track mid-clear.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/test_helpers/mock_player_channels.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+/// Installs mock method/event channel handlers for a native player for the
+/// duration of [testBody], then removes them.
+Future<void> withMockPlayerChannels({
+  required String methodChannelName,
+  required String eventChannelName,
+  Future<Object?> Function(MethodCall call)? methodHandler,
+  Future<Object?> Function(MethodCall call)? eventHandler,
+  required Future<void> Function() testBody,
+}) async {
+  final messenger = TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger;
+  final methodChannel = MethodChannel(methodChannelName);
+  final eventChannel = MethodChannel(eventChannelName);
+
+  messenger.setMockMethodCallHandler(
+    methodChannel,
+    methodHandler ??
+        (call) async {
+          switch (call.method) {
+            case 'initialize':
+              return true;
+            default:
+              return null;
+          }
+        },
+  );
+  messenger.setMockMethodCallHandler(eventChannel, eventHandler ?? (call) async => null);
+
+  try {
+    await testBody();
+  } finally {
+    messenger.setMockMethodCallHandler(methodChannel, null);
+    messenger.setMockMethodCallHandler(eventChannel, null);
+  }
+}