fix: agent chat completions can not use (#16570)

### Summary

As title
<img width="2370" height="2039" alt="image"
src="https://github.com/user-attachments/assets/4cccf543-3908-49ee-8101-c5068fbf53ec"
/>

internal/service/bot_completion.go

@@ -154,30 +154,22 @@ func WriteDoneFrame(w http.ResponseWriter) error {
 	return nil
 }
 
-// WriteChatbotRunEvent translates one canvas.RunEvent into the
-// unified python-shaped chat-completion envelope (same shape as
-// WriteChatbotFrame). This unifies the SSE format across:
+// WriteChatbotRunEvent translates one canvas.RunEvent into the flat
+// Python agent-canvas SSE envelope:
 //
-//   - /api/v1/agents/chat/completions   (was: writeChatCompletionSSE)
-//   - /api/v1/agentbots/<id>/completions (was: WriteChatbotFrame per-event)
+//	data: {"event":"message","message_id":"...","task_id":"...",
+//	  "session_id":"...","created_at":123,"data":{"content":"..."}}\n\n
+//
+// This is intentionally different from WriteChatbotFrame's legacy
+// chatbot `{code,data:{answer:"..."}}` shape. The agent React page's
+// use-send-message.ts parser appends each parsed object directly to
+// answerList and expects top-level `event` / `message_id`, plus a
+// typed `data` payload. If RunEvent frames are double-wrapped in
+// data.answer, the browser receives bytes but cannot render the
+// assistant message or correlate the current Log panel.
 //
 // The "done" event type emits `data: [DONE]\n\n` (no envelope),
-// matching the OpenAI-style terminator and the existing
-// AgentbotCompletion wire.
-//
-// For non-done events, ev.Data is placed verbatim into the `answer`
-// field — callers pass canvas-runner output that is itself a JSON
-// string (e.g. `{"answer":"hi back","reference":[]}`); the iframe
-// SDK then JSON.parse()s the `answer` string to extract the inner
-// fields. This matches the existing AgentbotCompletion behaviour.
-//
-// The event type is forwarded as the `event` field of the envelope
-// (PR #14589) so the front-end can distinguish interactive
-// `user_inputs` / `workflow_finished` events from plain `message`
-// streams and render the UserFillUp form vs the assistant text.
-// Without this field the form UI never appears because the
-// iframe SDK has no way to know the canvas paused for human
-// input.
+// matching the Python agent API terminator.
 //
 // Returns the write error so callers can short-circuit; both nil
 // and io.ErrClosedPipe are tolerated because the client may have
@@ -193,13 +185,65 @@ func WriteChatbotRunEvent(w http.ResponseWriter, ev canvas.RunEvent) error {
 		}
 		return nil
 	}
-	f := ChatbotSSEFrame{
-		Event:     ev.Type,
-		Data:      ev.Data,
-		Reference: map[string]any{},
-		SessionID: ev.SessionID,
+
+	var data any = map[string]any{}
+	if ev.Data != "" {
+		if err := json.Unmarshal([]byte(ev.Data), &data); err != nil {
+			data = ev.Data
+		}
 	}
-	return WriteChatbotFrame(w, f)
+	if ev.Type == "error" {
+		msg := "an internal error occurred"
+		if m, ok := data.(map[string]any); ok {
+			if s, _ := m["message"].(string); s != "" {
+				msg = s
+			}
+		}
+		payload := map[string]any{
+			"code":    500,
+			"message": msg,
+			"data":    false,
+		}
+		return writeSSEJSON(w, payload)
+	}
+
+	payload := map[string]any{
+		"data":       data,
+		"created_at": ev.CreatedAt,
+	}
+	if ev.Type != "" {
+		payload["event"] = ev.Type
+	}
+	if ev.MessageID != "" {
+		payload["message_id"] = ev.MessageID
+	}
+	if ev.TaskID != "" {
+		payload["task_id"] = ev.TaskID
+	}
+	if ev.SessionID != "" {
+		payload["session_id"] = ev.SessionID
+	}
+	return writeSSEJSON(w, payload)
+}
+
+func writeSSEJSON(w http.ResponseWriter, payload map[string]any) error {
+	b, err := runtime.SafeJSONMarshal(payload)
+	if err != nil {
+		return err
+	}
+	if _, err := w.Write([]byte("data:")); err != nil {
+		return err
+	}
+	if _, err := w.Write(b); err != nil {
+		return err
+	}
+	if _, err := w.Write([]byte("\n\n")); err != nil {
+		return err
+	}
+	if flusher, ok := w.(http.Flusher); ok {
+		flusher.Flush()
+	}
+	return nil
 }
 
 // AgentbotSSEFrame mirrors ChatbotSSEFrame for the agentbot