fix: support message content (#16980)

### Summary

As Title
<img width="3241" height="1934" alt="image"
src="https://github.com/user-attachments/assets/cb22031c-f200-462a-b871-b4a739e7a66a"
/>

internal/handler/agent.go

@@ -402,7 +402,11 @@ func (h *AgentHandler) RunAgent(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")
+	doneSent := false
 	for ev := range events {
+		if ev.Type == "done" {
+			doneSent = true
+		}
 		if err := service.WriteChatbotRunEvent(c.Writer, ev); err != nil {
 			common.Debug("agent run: client disconnected",
 				zap.String("canvas_id", canvasID),
@@ -412,6 +416,15 @@ func (h *AgentHandler) RunAgent(c *gin.Context) {
 			return
 		}
 	}
+	if !doneSent {
+		if err := service.WriteChatbotRunEvent(c.Writer, canvas.RunEvent{Type: "done"}); err != nil {
+			common.Debug("agent run: failed to write [DONE]",
+				zap.String("canvas_id", canvasID),
+				zap.String("session_id", sessionID),
+				zap.Error(err),
+			)
+		}
+	}
 }
 
 // readUserInput extracts the user_input field from the JSON body if
@@ -813,7 +826,7 @@ func (h *AgentHandler) DeleteAgentSession(c *gin.Context) {
 // api/apps/restful_apis/agent_api.py:1440-1676.
 //
 //   - When `stream` is true: streams SSE — one `data: {...}\n\n` frame per
-//     canvas RunEvent, terminated by `data: [DONE]\n\n`.
+//     canvas RunEvent, terminated by `data:[DONE]\n\n`.
 //   - When `stream` is omitted or false (default, matches the Python
 //     agent_chat_completion contract where `req.get("stream", False)`
 //     defaults to non-streaming): collects all canvas events and returns a
@@ -1008,7 +1021,7 @@ func (h *AgentHandler) AgentChatCompletions(c *gin.Context) {
 
 	if req.Stream {
 		// SSE streaming: one `data: {...}\n\n` frame per canvas RunEvent,
-		// terminated by `data: [DONE]\n\n`. We do NOT emit an SSE `event:`
+		// terminated 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 expects the event type
 		// in the JSON object's top-level `event` field.
@@ -1016,8 +1029,12 @@ func (h *AgentHandler) AgentChatCompletions(c *gin.Context) {
 		c.Writer.Header().Set("Cache-Control", "no-cache")
 		c.Writer.Header().Set("Connection", "keep-alive")
 		emitted := false
+		doneSent := false
 		for ev := range events {
 			emitted = true
+			if ev.Type == "done" {
+				doneSent = true
+			}
 			common.Debug("agent chat completions: streaming event",
 				zap.String("agent_id", req.AgentID),
 				zap.String("session_id", req.SessionID),
@@ -1037,7 +1054,7 @@ func (h *AgentHandler) AgentChatCompletions(c *gin.Context) {
 			// Canvas produced no events (e.g. empty query). Echo the
 			// session_id so the client can resume the conversation
 			// (fixes #15169). The [DONE] terminator must be emitted
-			// here explicitly because the canvas never sends a
+			// after this branch because the canvas never sends a
 			// "done" event on this path.
 			common.Info("empty agent output - returning session_id",
 				zap.String("agent_id", req.AgentID),
@@ -1050,7 +1067,9 @@ func (h *AgentHandler) AgentChatCompletions(c *gin.Context) {
 				SessionID: req.SessionID,
 			}
 			_ = service.WriteChatbotRunEvent(c.Writer, event)
-			if _, err := c.Writer.Write([]byte("data: [DONE]\n\n")); err != nil {
+		}
+		if !doneSent {
+			if err := service.WriteChatbotRunEvent(c.Writer, canvas.RunEvent{Type: "done"}); err != nil {
 				common.Debug("agent chat completions: failed to write [DONE]",
 					zap.Error(err),
 				)

internal/handler/agent_test.go

@@ -708,7 +708,7 @@ func TestAgentChatCompletions_OpenAICompat_EmptyMessages(t *testing.T) {
 // SSE tests. It emits a pre-configured sequence of canvas.RunEvent
 // values on its RunAgent channel and then closes — enough to verify
 // the SSE wire format (Content-Type, one `data: {...}\n\n` frame per
-// event, trailing `data: [DONE]\n\n`) without standing up the eino
+// event, trailing `data:[DONE]\n\n`) without standing up the eino
 // runner or a live DB.
 type stubChatRunner struct {
 	events []canvas.RunEvent
@@ -729,7 +729,7 @@ 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.
+// `data: {...}\n\n` with a trailing `data:[DONE]\n\n` terminator.
 // The frame shape is the Python agent-canvas envelope
 // {event,message_id,task_id,session_id,data:{content}}. See
 // service.WriteChatbotRunEvent.
@@ -765,7 +765,58 @@ func TestAgentChatCompletions_StreamSetsContentType(t *testing.T) {
 		!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") {
+	if !strings.HasSuffix(body, "data:[DONE]\n\n") {
+		t.Errorf("body should end with [DONE] terminator, got %q", body)
+	}
+}
+
+func TestAgentChatCompletions_StreamAddsDoneWhenRunnerCloses(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("POST", "/api/v1/agents/chat/completions",
+		strings.NewReader(`{"agent_id":"a1","stream":true,"query":"hi"}`))
+	c.Request.Header.Set("Content-Type", "application/json")
+	c.Set("user", &entity.User{ID: "u1"})
+	c.Set("user_id", "u1")
+
+	runner := &stubChatRunner{events: []canvas.RunEvent{
+		{Type: "message", MessageID: "msg-1", TaskID: "task-1", SessionID: "sess-1", Data: `{"content":"hi back"}`},
+	}}
+	h := &AgentHandler{chatRunner: runner}
+	h.AgentChatCompletions(c)
+
+	body := w.Body.String()
+	if got := strings.Count(body, "data:[DONE]\n\n"); got != 1 {
+		t.Fatalf("expected exactly one [DONE] terminator, got %d in %q", got, body)
+	}
+	if !strings.HasSuffix(body, "data:[DONE]\n\n") {
+		t.Errorf("body should end with [DONE] terminator, got %q", body)
+	}
+}
+
+func TestRunAgent_StreamAddsDoneWhenRunnerCloses(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "canvas_id", Value: "a1"}}
+	c.Request = httptest.NewRequest("POST", "/api/v1/agents/a1/run",
+		strings.NewReader(`{"user_input":"hi"}`))
+	c.Request.Header.Set("Content-Type", "application/json")
+	c.Set("user", &entity.User{ID: "u1"})
+	c.Set("user_id", "u1")
+
+	runner := &stubChatRunner{events: []canvas.RunEvent{
+		{Type: "message", MessageID: "msg-1", TaskID: "task-1", SessionID: "sess-1", Data: `{"content":"hi back"}`},
+	}}
+	h := &AgentHandler{chatRunner: runner}
+	h.RunAgent(c)
+
+	body := w.Body.String()
+	if got := strings.Count(body, "data:[DONE]\n\n"); got != 1 {
+		t.Fatalf("expected exactly one [DONE] terminator, got %d in %q", got, body)
+	}
+	if !strings.HasSuffix(body, "data:[DONE]\n\n") {
 		t.Errorf("body should end with [DONE] terminator, got %q", body)
 	}
 }
@@ -804,7 +855,7 @@ func TestAgentChatCompletions_DefaultBranchNonStreaming(t *testing.T) {
 		!strings.Contains(body, `"hello back"`) {
 		t.Errorf("body should contain agent event with content in data, got %q", body)
 	}
-	if strings.Contains(body, "data: [DONE]") {
+	if strings.Contains(body, "data:[DONE]") {
 		t.Errorf("body should not contain [DONE] terminator in non-streaming mode, got %q", body)
 	}
 }