refactor: replace assert with natural type narrowing for null-id check

Restructure the null-id guard so that `id is None` is checked first,
allowing Pyright to naturally narrow the type to JSONRPCError (since
JSONRPCResponse.id is always RequestId). This eliminates the need for
the assert that was previously required to work around Pyright's
inability to narrow through negated compound conditions.

Author: maxisbey

src/mcp/shared/session.py

@@ -458,19 +458,12 @@ async def _handle_response(self, message: SessionMessage) -> None:
         if not isinstance(message.message, JSONRPCResponse | JSONRPCError):
             return  # pragma: no cover
 
-        # Handle null-id errors (e.g., parse errors, invalid requests) before
-        # routing. Per JSON-RPC 2.0, id is null when the request id could not
-        # be determined — these cannot be correlated to any pending request.
-        if isinstance(message.message, JSONRPCError) and message.message.id is None:
+        if message.message.id is None:
+            # Narrows to JSONRPCError since JSONRPCResponse.id is always RequestId
             error = message.message.error
             logging.warning(f"Received error with null ID: {error.message}")
             await self._handle_incoming(MCPError(error.code, error.message, error.data))
             return
-
-        # After the null-id guard above, id is guaranteed to be non-None.
-        # JSONRPCResponse.id is always RequestId, and JSONRPCError.id is only
-        # None for parse errors (handled above).
-        assert message.message.id is not None
         # Normalize response ID to handle type mismatches (e.g., "0" vs 0)
         response_id = self._normalize_request_id(message.message.id)