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

internal/handler/agent_test.go

@@ -730,10 +730,9 @@ func (s *stubChatRunner) RunAgent(_ context.Context, _, _, _, _ string, _ any) (
 // TestAgentChatCompletions_StreamSetsContentType covers the SSE
 // path: the handler streams canvas.RunEvent frames as
 // `data: {...}\n\n` with a trailing `data: [DONE]\n\n` terminator.
-// The frame shape is the unified python envelope
-// {code:0, message:"", data:{answer, reference, audio_binary, id,
-// session_id}} — the same shape /api/v1/agentbots/<id>/completions
-// emits. See service.WriteChatbotRunEvent and WriteChatbotFrame.
+// The frame shape is the Python agent-canvas envelope
+// {event,message_id,task_id,session_id,data:{content}}. See
+// service.WriteChatbotRunEvent.
 //
 // The stubChatRunner emits one `message` frame and one `done` frame
 // so the test verifies the body contains both the framed event and
@@ -749,7 +748,7 @@ func TestAgentChatCompletions_StreamSetsContentType(t *testing.T) {
 	c.Set("user_id", "u1")
 
 	runner := &stubChatRunner{events: []canvas.RunEvent{
-		{Type: "message", Data: `{"answer":"hi back","reference":[]}`},
+		{Type: "message", MessageID: "msg-1", TaskID: "task-1", SessionID: "sess-1", Data: `{"content":"hi back","reference":[]}`},
 		{Type: "done", Data: ""},
 	}}
 	h := &AgentHandler{chatRunner: runner}
@@ -759,12 +758,12 @@ func TestAgentChatCompletions_StreamSetsContentType(t *testing.T) {
 		t.Errorf("Content-Type = %q, want text/event-stream", got)
 	}
 	body := w.Body.String()
-	// Body must contain the unified python envelope (`code/data.answer`)
-	// and the [DONE] terminator. The iframe SDK JSON.parse()s `answer`
-	// to extract the inner fields, so the embedded JSON is double-encoded
-	// (escaped quotes inside the outer `"answer"` string).
-	if !strings.Contains(body, "\"code\":0") || !strings.Contains(body, `"answer":"{\"answer\":\"hi back\",\"reference\":[]}"`) {
-		t.Errorf("body should contain unified python envelope with answer, got %q", body)
+	if !strings.Contains(body, `"event":"message"`) ||
+		!strings.Contains(body, `"message_id":"msg-1"`) ||
+		!strings.Contains(body, `"task_id":"task-1"`) ||
+		!strings.Contains(body, `"session_id":"sess-1"`) ||
+		!strings.Contains(body, `"content":"hi back"`) {
+		t.Errorf("body should contain flat agent event with content, got %q", body)
 	}
 	if !strings.HasSuffix(body, "data: [DONE]\n\n") {
 		t.Errorf("body should end with [DONE] terminator, got %q", body)
@@ -775,8 +774,7 @@ func TestAgentChatCompletions_StreamSetsContentType(t *testing.T) {
 // scenario the user actually hit: `openai-compatible: false` with no
 // `stream` field on the body. The handler must still invoke the
 // canvas runner and stream the result as SSE — the SSE envelope is
-// the unified python shape shared with
-// /api/v1/agentbots/<id>/completions regardless of the stream flag.
+// the flat Python agent-canvas shape regardless of the stream flag.
 func TestAgentChatCompletions_DefaultBranchStreamsSSE(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	w := httptest.NewRecorder()
@@ -788,7 +786,7 @@ func TestAgentChatCompletions_DefaultBranchStreamsSSE(t *testing.T) {
 	c.Set("user_id", "u1")
 
 	runner := &stubChatRunner{events: []canvas.RunEvent{
-		{Type: "message", Data: `{"answer":"hello back","reference":[]}`},
+		{Type: "message", MessageID: "msg-2", TaskID: "task-2", SessionID: "sess-2", Data: `{"content":"hello back","reference":[]}`},
 		{Type: "done", Data: ""},
 	}}
 	h := &AgentHandler{chatRunner: runner}
@@ -798,8 +796,10 @@ func TestAgentChatCompletions_DefaultBranchStreamsSSE(t *testing.T) {
 		t.Errorf("Content-Type = %q, want text/event-stream (default branch must stream)", got)
 	}
 	body := w.Body.String()
-	if !strings.Contains(body, "\"code\":0") || !strings.Contains(body, `"answer":"{\"answer\":\"hello back\",\"reference\":[]}"`) {
-		t.Errorf("body should contain unified python envelope with answer, got %q", body)
+	if !strings.Contains(body, `"event":"message"`) ||
+		!strings.Contains(body, `"message_id":"msg-2"`) ||
+		!strings.Contains(body, `"content":"hello back"`) {
+		t.Errorf("body should contain flat agent event with content, got %q", body)
 	}
 	if !strings.HasSuffix(body, "data: [DONE]\n\n") {
 		t.Errorf("body should end with [DONE] terminator, got %q", body)