Fix websocket error responses in Mux dispatcher (#726)

Error responses from the websocket multiplexer were missing the
request ID and using a bare string format instead of the structured
error protocol. This caused clients to hang when a request failed
(e.g. unsupported service for a flow) because the error could not
be routed to the waiting caller.

Include request ID in all error paths, use structured error format
({message, type}) with complete flag, and extract the ID early in
receive() so even malformed requests get a routable error when
possible.

Updated tests - tests were coded against invalid protocol messages

tests/unit/test_gateway/test_dispatch_mux.py

@@ -121,7 +121,11 @@ class TestMux:
         # Based on the code, it seems to catch exceptions
         await mux.receive(mock_msg)
         
-        mock_ws.send_json.assert_called_once_with({"error": "Bad message"})
+        mock_ws.send_json.assert_called_once_with({
+            "error": {"message": "Bad message", "type": "error"},
+            "complete": True,
+            "id": "test-id-123",
+        })
 
     @pytest.mark.asyncio
     async def test_mux_receive_message_without_id(self):
@@ -145,7 +149,10 @@ class TestMux:
         # receive method should handle the RuntimeError internally
         await mux.receive(mock_msg)
 
-        mock_ws.send_json.assert_called_once_with({"error": "Bad message"})
+        mock_ws.send_json.assert_called_once_with({
+            "error": {"message": "Bad message", "type": "error"},
+            "complete": True,
+        })
 
     @pytest.mark.asyncio
     async def test_mux_receive_invalid_json(self):
@@ -168,4 +175,7 @@ class TestMux:
         await mux.receive(mock_msg)
 
         mock_msg.json.assert_called_once()
-        mock_ws.send_json.assert_called_once_with({"error": "Invalid JSON"})
+        mock_ws.send_json.assert_called_once_with({
+            "error": {"message": "Invalid JSON", "type": "error"},
+            "complete": True,
+        })

trustgraph-flow/trustgraph/gateway/dispatch/mux.py

@@ -33,9 +33,12 @@ class Mux:
 
     async def receive(self, msg):
 
+        request_id = None
+
         try:
 
             data = msg.json()
+            request_id = data.get("id")
 
             if "request" not in data:
                 raise RuntimeError("Bad message")
@@ -51,7 +54,13 @@ class Mux:
 
         except Exception as e:
             logger.error(f"Receive exception: {str(e)}", exc_info=True)
-            await self.ws.send_json({"error": str(e)})
+            error_resp = {
+                "error": {"message": str(e), "type": "error"},
+                "complete": True,
+            }
+            if request_id:
+                error_resp["id"] = request_id
+            await self.ws.send_json(error_resp)
 
     async def maybe_tidy_workers(self, workers):
 
@@ -97,12 +106,12 @@ class Mux:
             })
 
         worker = asyncio.create_task(
-            self.request_task(request, responder, flow, svc)
+            self.request_task(id, request, responder, flow, svc)
         )
 
         workers.append(worker)
 
-    async def request_task(self, request, responder, flow, svc):
+    async def request_task(self, id, request, responder, flow, svc):
 
         try:
 
@@ -119,7 +128,11 @@ class Mux:
                 )
 
         except Exception as e:
-            await self.ws.send_json({"error": str(e)})
+            await self.ws.send_json({
+                "id": id,
+                "error": {"message": str(e), "type": "error"},
+                "complete": True,
+            })
 
     async def run(self):
 
@@ -143,7 +156,11 @@ class Mux:
             except Exception as e:
                 # This is an internal working error, may not be recoverable
                 logger.error(f"Run prepare exception: {e}", exc_info=True)
-                await self.ws.send_json({"id": id, "error": str(e)})
+                await self.ws.send_json({
+                    "id": id,
+                    "error": {"message": str(e), "type": "error"},
+                    "complete": True,
+                })
                 self.running.stop()
 
                 if self.ws:
@@ -160,7 +177,11 @@ class Mux:
 
             except Exception as e:
                 logger.error(f"Exception in mux: {e}", exc_info=True)
-                await self.ws.send_json({"error": str(e)})
+                await self.ws.send_json({
+                    "id": id,
+                    "error": {"message": str(e), "type": "error"},
+                    "complete": True,
+                })
 
         self.running.stop()