FN-7101: stabilize App deep-link history state

Reset App deep-link test history state around replaceState stubs so full-file order no longer leaks navigation state.

- Reset the real browser history snapshot before stubbing replaceState.
- Restore replaceState before cleanup so history state and URL are reset through the original implementation.
- Document why deep-link tests must clear navIndex between mounts.

Files changed:
 packages/dashboard/app/components/__tests__/App.test.tsx | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-7101

Fusion-Task-Lineage: 5c44441f-f0d2-4ad3-bdcf-5a0a0e3e6e6f

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

packages/dashboard/app/components/__tests__/App.test.tsx

@@ -1411,6 +1411,11 @@ describe("App deep link handling", () => {
   const originalReplaceState = window.history.replaceState;
 
   beforeEach(() => {
+    /*
+     * FNXC:DashboardTests 2026-06-26-23:40:
+     * Deep-link App tests stub `history.replaceState` for assertions, but `useNavigationHistory` still writes real `pushState` entries and preserves existing `history.state.navIndex`. Reset the real history snapshot before installing the stub so full-file ordering cannot mount a later Back test with stale browser state.
+     */
+    originalReplaceState.call(window.history, null, "", "/");
     window.history.replaceState = vi.fn();
     Object.defineProperty(window, "location", {
       configurable: true,
@@ -1419,11 +1424,12 @@ describe("App deep link handling", () => {
   });
 
   afterEach(() => {
+    window.history.replaceState = originalReplaceState;
+    originalReplaceState.call(window.history, null, "", "/");
     Object.defineProperty(window, "location", {
       configurable: true,
       value: originalLocation,
     });
-    window.history.replaceState = originalReplaceState;
   });
 
   it("fetches and opens the task modal when task query param is present", async () => {