feat(FN-3981): document room message resync contract and reproduce reply vi

Fixes a room reply visibility gap in the chat rooms hook by correcting the message-resync condition and adding test coverage to reproduce the issue, with updated documentation for the resync contract.

Fusion-Task-Id: FN-3981

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

@@ -182,7 +182,7 @@ describe("useChatRooms", () => {
     expect(result.current.activeRoom).toBeNull();
   });
 
-  it("sendRoomMessage posts without optimistic insert", async () => {
+  it("sendRoomMessage resyncs room messages from server after post", async () => {
     const active = room("room-1", "one", "2026-05-09T01:00:00.000Z");
     mockFetchChatRooms.mockResolvedValueOnce({ rooms: [active] });
     const { result } = renderHook(() => useChatRooms("proj-1"));
@@ -193,17 +193,20 @@ describe("useChatRooms", () => {
     act(() => result.current.selectRoom("room-1"));
     await waitFor(() => expect(result.current.activeRoom?.id).toBe("room-1"));
 
+    mockFetchChatRoomMessages.mockResolvedValueOnce({
+      messages: [
+        roomMessage("msg-user", "room-1", "hello"),
+        { ...roomMessage("msg-assistant", "room-1", "Room reply"), role: "assistant", senderAgentId: "agent-1" },
+      ],
+    });
+
     await act(async () => {
       await result.current.sendRoomMessage("hello");
     });
 
     expect(mockPostChatRoomMessage).toHaveBeenCalledWith("room-1", { content: "hello" }, "proj-1");
-    expect(result.current.messages).toHaveLength(0);
-
-    act(() => {
-      capturedEvents["chat:room:message:added"]?.({ data: JSON.stringify(roomMessage("msg-1", "room-1", "hello")) } as MessageEvent);
-    });
-    expect(result.current.messages).toHaveLength(1);
+    expect(mockFetchChatRoomMessages).toHaveBeenLastCalledWith("room-1", { limit: 100 }, "proj-1");
+    expect(result.current.messages.map((message) => message.id)).toEqual(["msg-user", "msg-assistant"]);
   });
 
   it("tears down sse subscription on unmount", async () => {

packages/dashboard/app/hooks/useChatRooms.ts

@@ -168,15 +168,26 @@ export function useChatRooms(
   }, [projectId]);
 
   const sendRoomMessage = useCallback(async (content: string, opts?: { attachments?: ChatAttachment[] }) => {
-    const roomId = activeRoomRef.current?.id;
+    const activeRoomSnapshot = activeRoomRef.current;
+    const roomId = activeRoomSnapshot?.id;
     if (!roomId) {
       throw new Error("Select a room before sending a message");
     }
 
-    await postChatRoomMessage(roomId, {
+    const postResult = await postChatRoomMessage(roomId, {
       content,
       ...(opts?.attachments ? { attachments: opts.attachments } : {}),
     }, projectId);
+
+    if (postResult.message?.createdAt && activeRoomSnapshot) {
+      setRooms((previous) => upsertRoom(previous, { ...activeRoomSnapshot, updatedAt: postResult.message.createdAt }));
+    }
+
+    const latestMessages = await fetchChatRoomMessages(roomId, { limit: 100 }, projectId);
+    if (activeRoomRef.current?.id !== roomId) {
+      return;
+    }
+    setMessages(latestMessages.messages);
   }, [projectId]);
 
   useEffect(() => {