fix(FN-3760): add SSE first-event watchdog for legacy chat streams

- Add a first-event watchdog in legacy chat SSE handling to prevent hangs before initial stream data
- Cover watchdog behavior with legacy chat stream API tests
- Add hook-level regression tests for useChat and useQuickChat first-send stream scenarios
- Include a changeset for @runfusion/fusion patch release

Fusion-Task-Id: FN-3760

packages/dashboard/app/hooks/__tests__/useChat.test.ts

@@ -456,6 +456,46 @@ describe("useChat", () => {
     });
   });
 
+  it("sets isStreaming true during first send and clears on delayed done", async () => {
+    const session = makeSession({
+      id: "session-001",
+      agentId: "agent-001",
+      title: "Test Session",
+    });
+
+    mockFetchChatSessions.mockResolvedValue({ sessions: [session] });
+    mockFetchChatMessages.mockResolvedValue({ messages: [] });
+
+    const { result } = renderHook(() => useChat(undefined, "project-123"));
+
+    await waitFor(() => {
+      expect(result.current.sessions).toHaveLength(1);
+    });
+
+    act(() => {
+      result.current.selectSession("session-001");
+    });
+
+    mockStreamChatResponse.mockImplementation((_sessionId, _content, handlers) => {
+      setTimeout(() => {
+        handlers.onDone?.({ messageId: "msg-001" });
+      }, 200);
+      return { close: vi.fn(), isConnected: () => true };
+    });
+
+    act(() => {
+      void result.current.sendMessage("Hello");
+    });
+
+    await waitFor(() => {
+      expect(result.current.isStreaming).toBe(true);
+    });
+
+    await waitFor(() => {
+      expect(result.current.isStreaming).toBe(false);
+    });
+  });
+
   it("uses done payload assistant snapshot when no text chunks were streamed", async () => {
     const session = makeSession({ id: "session-001", agentId: "agent-001" });
     mockFetchChatSessions.mockResolvedValueOnce({ sessions: [session] });

packages/dashboard/app/hooks/__tests__/useQuickChat.test.ts

@@ -78,6 +78,37 @@ describe("useQuickChat", () => {
     await expect(sendResult).resolves.toBeUndefined();
   });
 
+  it("sets isStreaming true during first send and clears on delayed done", async () => {
+    const session = makeSession({ id: "session-001", agentId: "agent-001" });
+    mockFetchResumeChatSession.mockResolvedValue({ session });
+    mockFetchChatMessages.mockResolvedValue({ messages: [] });
+
+    const { result } = renderHook(() => useQuickChat("proj-123"));
+
+    await act(async () => {
+      await result.current.switchSession("agent-001");
+    });
+
+    mockStreamChatResponse.mockImplementation((_sessionId, _content, handlers) => {
+      setTimeout(() => {
+        handlers.onDone?.({ messageId: "msg-001" });
+      }, 200);
+      return { close: vi.fn(), isConnected: () => true };
+    });
+
+    void act(() => {
+      void result.current.sendMessage("Hello");
+    });
+
+    await waitFor(() => {
+      expect(result.current.isStreaming).toBe(true);
+    });
+
+    await waitFor(() => {
+      expect(result.current.isStreaming).toBe(false);
+    });
+  });
+
   it("uses done payload assistant snapshot when no text chunks were streamed", async () => {
     const session = makeSession({ id: "session-001", agentId: "agent-001" });
     mockFetchResumeChatSession.mockResolvedValue({ session });