feat (core): If user_format is provided, map it to the correct container if it's a codec name

audio_splitter/formats.py

@@ -47,7 +47,7 @@ def determine_output_format(stream_info: Dict, user_format: Optional[str],
     """
     Decide which container format to use.
 
-    If user_format is provided, use it.
+    If user_format is provided, map it to the correct container if it's a codec name.
     Else, try to detect the input file's container and codec, and use the recommended format.
     If detection fails or format is not supported, fallback to:
       - MKV if video/subtitles exist
@@ -55,6 +55,10 @@ def determine_output_format(stream_info: Dict, user_format: Optional[str],
       - MP4 (M4A) otherwise
     """
     if user_format:
+        # Map codec names to their correct containers
+        # e.g., 'opus' -> 'ogg', 'aac' -> 'mp4', etc.
+        if user_format in CODEC_TO_CONTAINER_MAP:
+            return CODEC_TO_CONTAINER_MAP[user_format]
         return user_format
 
     if input_file: