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.

android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt

@@ -501,6 +501,8 @@ object NativeDownloadFinalizer {
         shouldCancel: () -> Boolean,
     ) {
         if (requestQuality(input) == "HIGH" || outputExt(input) != ".flac") return
+        val requestedDecryptionExt = requestedDecryptionOutputExt(input)
+        if (requestedDecryptionExt.isNotBlank() && requestedDecryptionExt != ".flac") return
         if (!looksLikeM4a(state.filePath, state.fileName) && !shouldForceContainerConversion(input, state)) return
 
         val localInput = materializeForFFmpeg(context, input, state)
@@ -1330,6 +1332,14 @@ object NativeDownloadFinalizer {
         return container == "m4a" || container == "mp4" || container == "mov" || container == "aac"
     }
 
+    private fun requestedDecryptionOutputExt(input: FinalizeInput): String {
+        val descriptor = input.result.optJSONObject("decryption")
+        return normalizeExt(
+            descriptor?.optString("output_extension", "")
+                ?.ifBlank { input.result.optString("output_extension", "") }
+        )
+    }
+
     private fun validateRequestContract(request: JSONObject) {
         val version = request.optInt("contract_version", -1)
         if (version != NATIVE_WORKER_CONTRACT_VERSION) {