chore: add TODO comment for error code alignment

Link to tracking issue for JSON-RPC error code clarification.

Author: maxisbey

src/mcp/server/streamable_http_manager.py

@@ -279,6 +279,8 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                 await http_transport.handle_request(scope, receive, send)
         else:
             # Unknown or expired session ID - return 404 per MCP spec
+            # TODO: Align error code once spec clarifies
+            # See: https://github.com/modelcontextprotocol/python-sdk/issues/1821
             error_response = JSONRPCError(
                 jsonrpc="2.0",
                 id="server-error",