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/handler/agent.go

@@ -1094,19 +1094,14 @@ func (h *AgentHandler) AgentChatCompletions(c *gin.Context) {
 	c.Writer.Header().Set("Content-Type", "text/event-stream")
 	c.Writer.Header().Set("Cache-Control", "no-cache")
 	c.Writer.Header().Set("Connection", "keep-alive")
-	// SSE wire format is the unified python envelope used by both
-	// /api/v1/agents/chat/completions and /api/v1/agentbots/<id>/completions.
-	// One frame per canvas event, all routed through
-	// service.WriteChatbotRunEvent so the two paths share one writer
-	// and one shape — see internal/service/bot_completion.go for the
-	// frame definition. The same unified envelope is used by the
-	// /api/v1/agents/{canvas_id}/run and /api/v1/agentbots/<id>/completions
-	// endpoints, all going through service.WriteChatbotRunEvent. The
-	// channel close is signalled by `data: [DONE]\n\n`. We do NOT emit
-	// an SSE `event:` line — the front-end's `use-send-message.ts`
+	// SSE wire format is the flat Python agent-canvas envelope:
+	// {event,message_id,task_id,session_id,created_at,data}. One
+	// frame is emitted per canvas event through service.WriteChatbotRunEvent.
+	// The channel close is signalled by `data: [DONE]\n\n`. We do NOT
+	// emit an SSE `event:` line — the front-end's use-send-message.ts
 	// parser feeds each `data:` line directly into JSON.parse and
-	// breaks on the `e` of `event:` (browser console: "SyntaxError:
-	// Unexpected token 'e', \"event: mes\"…").
+	// expects the event type in the JSON object's top-level `event`
+	// field.
 	emitted := false
 	for ev := range events {
 		emitted = true