fix(alerts): replace stale-anchor UI check with payload-level assertion

Trigger save after stale anchor is processed and inspect the PUT request
body to verify extra.dashboard.anchor is undefined. This directly asserts
the component state through its API boundary rather than checking DOM
elements or i18n-dependent UI copy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

superset-frontend/src/features/alerts/AlertReportModal.test.tsx

@@ -1222,12 +1222,29 @@ test('stale JSON array anchor is cleared without crash or toast', async () => {
       ),
     ).toBe(false);
 
-    // Verify anchor was cleared: the stale anchor value should not appear
-    // as a selected item anywhere (antd TreeSelect sets title={value} on
-    // selection items; absent title means updateAnchorState(undefined) ran)
+    // Verify anchor was cleared at the payload level: trigger save and
+    // inspect the PUT body to confirm extra.dashboard.anchor is undefined
+    const updateEndpoint = 'glob:*/api/v1/report/1';
+    fetchMock.put(updateEndpoint, { id: 1, result: {} }, { name: 'put-report-1' });
+
+    const saveButton = screen.getByRole('button', { name: /save/i });
+    expect(saveButton).not.toBeDisabled();
+    userEvent.click(saveButton);
+
     await waitFor(() => {
-      expect(screen.queryByTitle(staleAnchor)).not.toBeInTheDocument();
+      const putCalls = fetchMock.callHistory
+        .calls()
+        .filter(c => c.url.includes('/api/v1/report/') && c.options?.method === 'put');
+      expect(putCalls).toHaveLength(1);
     });
+
+    const putCall = fetchMock.callHistory
+      .calls()
+      .find(c => c.url.includes('/api/v1/report/') && c.options?.method === 'put');
+    const body = JSON.parse(putCall!.options?.body as string);
+    expect(body.extra.dashboard.anchor).toBeUndefined();
+
+    fetchMock.removeRoute('put-report-1');
   } finally {
     restoreAnchorMocks();
   }