feat(webui): track optimistic message delivery status (#5162)

webui/src/tests/thread-messages.test.tsx

@@ -954,6 +954,34 @@ describe("ThreadMessages", () => {
     expect(screen.getAllByRole("button", { name: "Copy" })).toHaveLength(2);
   });
 
+  it("does not count failed optimistic messages in assistant fork indices", () => {
+    const onForkFromMessage = vi.fn();
+    const messages: UIMessage[] = [
+      { id: "u1", role: "user", content: "one", createdAt: 1 },
+      { id: "a1", role: "assistant", content: "answer one", createdAt: 2 },
+      {
+        id: "u-failed",
+        role: "user",
+        content: "not persisted",
+        deliveryStatus: "failed",
+        createdAt: 3,
+      },
+      { id: "u2", role: "user", content: "two", createdAt: 4 },
+      { id: "a2", role: "assistant", content: "answer two", createdAt: 5 },
+    ];
+
+    render(
+      <ThreadMessages
+        messages={messages}
+        isStreaming={false}
+        onForkFromMessage={onForkFromMessage}
+      />,
+    );
+
+    fireEvent.click(screen.getAllByRole("button", { name: "Fork" }).at(-1)!);
+    expect(onForkFromMessage).toHaveBeenCalledWith(2);
+  });
+
   it("uses turn ids as activity grouping boundaries when available", () => {
     const units = buildDisplayUnits([
       { id: "u1", role: "user", content: "one", turnId: "turn-1", createdAt: 1 },