feat(FN-4835): complete Step 4 — prefer streamed accumulator on done

Fusion-Task-Id: FN-4835
Fusion-Task-Lineage: 09eb6339-a476-4cf1-b7e5-e60fa6c263ce

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

@@ -610,7 +610,7 @@ describe("useChat", () => {
     });
   });
 
-  it("prefers done payload assistant snapshot over streamed text", async () => {
+  it("prefers accumulated streamed text over done payload snapshot when both exist", async () => {
     const session = makeSession({ id: "session-001", agentId: "agent-001" });
     mockFetchChatSessions.mockResolvedValueOnce({ sessions: [session] });
     mockFetchChatMessages.mockResolvedValueOnce({ messages: [] });
@@ -633,16 +633,21 @@ describe("useChat", () => {
       result.current.selectSession("session-001");
     });
 
+    await waitFor(() => {
+      expect(result.current.activeSession?.id).toBe("session-001");
+    });
+
     act(() => {
       result.current.sendMessage("Hello!");
-      textHandler?.("streamed text");
+      textHandler?.("Hello.");
+      textHandler?.(" World.");
       doneHandler?.({
         messageId: "msg-003",
         message: {
           id: "msg-003",
           sessionId: "session-001",
           role: "assistant",
-          content: "snapshot wins",
+          content: "Hello.World.",
           thinkingOutput: null,
           metadata: null,
           createdAt: "2026-01-01T00:00:00.000Z",
@@ -653,7 +658,7 @@ describe("useChat", () => {
     await waitFor(() => {
       expect(result.current.messages.at(-1)).toEqual(expect.objectContaining({
         id: "msg-003",
-        content: "snapshot wins",
+        content: "Hello. World.",
       }));
     });
   });

packages/dashboard/app/hooks/useChat.ts

@@ -734,7 +734,12 @@ export function useChat(
         },
         onDone: ({ messageId, message: finalMessage, accumulated }) => {
           const assistantMessage: ChatMessageInfo = finalMessage
-            ? mapChatMessageToInfo(finalMessage)
+            ? {
+                ...mapChatMessageToInfo(finalMessage),
+                // FN-4835 (downstream of FN-3817): the streamed accumulator is
+                // the authoritative wire transcript, so keep it when present.
+                ...(accumulated.text.length > 0 ? { content: accumulated.text } : {}),
+              }
             : {
                 id: messageId || `msg-${Date.now()}`,
                 sessionId: activeSession.id,

packages/dashboard/app/hooks/useQuickChat.ts

@@ -738,7 +738,12 @@ export function useQuickChat(
           },
           onDone: ({ messageId, message: finalMessage, accumulated }) => {
             const assistantMessage: ChatMessageInfo = finalMessage
-              ? mapChatMessageToInfo(finalMessage)
+              ? {
+                  ...mapChatMessageToInfo(finalMessage),
+                  // FN-4835 (downstream of FN-3817): keep wire-accurate streamed
+                  // text when available instead of potentially lossy snapshots.
+                  ...(accumulated.text.length > 0 ? { content: accumulated.text } : {}),
+                }
               : {
                   id: messageId || `msg-${Date.now()}`,
                   sessionId: activeSession.id,