fix(native-worker): preserve requested output container in finalizer

When the native worker result advertises a requested non-FLAC output extension (for example '.m4a'), skip the m4a-to-flac container conversion in both the Dart and Kotlin finalizers so the native output container is preserved end-to-end.

- ffmpeg_service: propagate the top-level 'output_extension' hint into the download-result descriptor for both the map-backed and legacy paths; expose a normalized getter for consistent comparisons.

- download_queue_provider: short-circuit the native-worker container-conversion step when the descriptor's requested extension is not '.flac', with a debug log describing the skip.

- NativeDownloadFinalizer: mirror the guard on the Kotlin side so the finalizer does not force a container conversion that would clobber the requested native output.

lib/services/ffmpeg_service.dart

@@ -66,9 +66,9 @@ class DownloadDecryptionDescriptor {
   ) {
     final rawDecryption = result['decryption'];
     if (rawDecryption is Map) {
-      final descriptor = DownloadDecryptionDescriptor.fromJson(
-        Map<String, dynamic>.from(rawDecryption),
-      );
+      final descriptorJson = Map<String, dynamic>.from(rawDecryption);
+      descriptorJson['output_extension'] ??= result['output_extension'];
+      final descriptor = DownloadDecryptionDescriptor.fromJson(descriptorJson);
       if (descriptor.normalizedStrategy == 'ffmpeg.mov_key' &&
           descriptor.key.isNotEmpty) {
         return descriptor;
@@ -84,6 +84,7 @@ class DownloadDecryptionDescriptor {
       strategy: 'ffmpeg.mov_key',
       key: legacyKey,
       inputFormat: 'mov',
+      outputExtension: (result['output_extension'] as String?)?.trim(),
     );
   }
 
@@ -100,6 +101,12 @@ class DownloadDecryptionDescriptor {
         return strategy.trim();
     }
   }
+
+  String? get normalizedOutputExtension {
+    final trimmed = (outputExtension ?? '').trim().toLowerCase();
+    if (trimmed.isEmpty) return null;
+    return trimmed.startsWith('.') ? trimmed : '.$trimmed';
+  }
 }
 
 class FFmpegService {