Fix: Ensure ASGI compliance in transport.py for POST message handling (#162)

- Modified mcpm.router.transport.py:

  - Refactored handle_post_message to correctly send a 202 Accepted response as a raw ASGI application before proceeding with internal message forwarding. It now implicitly returns None as expected.

(Note: Complementary ASGI fixes for mcpm.router.router.py's handle_sse were already present in the base branch 076a6bf)

Author: iSevenDays

src/mcpm/router/transport.py

@@ -220,16 +220,16 @@ async def handle_post_message(self, scope: Scope, receive: Receive, send: Send):
             response = Response("Could not parse message", status_code=400)
             await response(scope, receive, send)
             try:
-                await writer.send(err)
+                await writer.send(SessionMessage(message=err))
             except (BrokenPipeError, ConnectionError, OSError) as pipe_err:
                 logger.warning(f"Failed to send error due to pipe issue: {pipe_err}")
             return
 
-        logger.debug(f"Sending message to writer: {message}")
-        response = Response("Accepted", status_code=202)
-        await response(scope, receive, send)
+        # Send the 202 Accepted response
+        accepted_response = Response("Accepted", status_code=202)
+        await accepted_response(scope, receive, send)
 
-        # add error handling, catch possible pipe errors
+        # Attempt to send the message to the writer
         try:
             await writer.send(SessionMessage(message=message))
         except (BrokenPipeError, ConnectionError, OSError) as e:
@@ -240,7 +240,9 @@ async def handle_post_message(self, scope: Scope, receive: Receive, send: Send):
                 logger.warning(f"Connection error when sending message to session {session_id}: {e}")
                 self._read_stream_writers.pop(session_id, None)
                 self._session_id_to_identifier.pop(session_id, None)
-        return response
+        
+        # Implicitly return None. The original 'return response' is removed.
+        return
 
     def _validate_api_key(self, scope: Scope, api_key: str | None) -> bool:
         # If api_key is explicitly set to None, disable API key validation