fix: Stream errors as simple JSON objects in ADK web server SSE

The ADK web server's /run_sse endpoint now yields a JSON object like {"error": "..."} when an exception occurs during event generation. The adk_web_server_client is updated to detect this error payload and raise a RuntimeError.

Close #4291

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 863475838

src/google/adk/cli/adk_web_server.py

@@ -1474,17 +1474,7 @@ class AdkWebServer:
                 yield f"data: {sse_event}\n\n"
         except Exception as e:
           logger.exception("Error in event_generator: %s", e)
-          # Yield a proper Event object for the error
-          error_event = Event(
-              author="system",
-              content=types.Content(
-                  role="model", parts=[types.Part(text=f"Error: {e}")]
-              ),
-          )
-          yield (
-              "data:"
-              f" {error_event.model_dump_json(by_alias=True, exclude_none=True)}\n\n"
-          )
+          yield f"data: {json.dumps({'error': str(e)})}\n\n"
 
       # Returns a streaming response with the proper media type for SSE
       return StreamingResponse(

src/google/adk/cli/conformance/adk_web_server_client.py

@@ -228,6 +228,7 @@ class AdkWebServerClient:
       ValueError: If mode is provided but test_case_dir or user_message_index is None
       httpx.HTTPStatusError: If the request fails
       json.JSONDecodeError: If event data cannot be parsed
+      RuntimeError: If the server streams an error payload
     """
     # Add recording parameters to state_delta for conformance tests
     if mode:
@@ -262,6 +263,8 @@ class AdkWebServerClient:
         async for line in response.aiter_lines():
           if line.startswith("data:") and (data := line[5:].strip()):
             event_data = json.loads(data)
+            if isinstance(event_data, dict) and "error" in event_data:
+              raise RuntimeError(event_data["error"])
             yield Event.model_validate(event_data)
           else:
             logger.debug("Non data line received: %s", line)