fix(test): Fix flaky MetricsTabContent sidebar toggle test

Wait for the SearchQueryBuilderCombobox inside the re-expanded sidebar
to finish initialising. Without this, its async state updates fire after
the test ends and trigger an act() warning that jest-fail-on-console
catches, causing flaky failures (4 occurrences in the last 30 days).

Co-Authored-By: Claude Sonnet 4 <noreply@example.com>
Made-with: Cursor

Author: JoshuaKGoldberg

static/app/views/explore/metrics/metricsTab.spec.tsx

@@ -731,5 +731,14 @@ describe('MetricsTabContent (tracemetrics-ui-refresh)', () => {
     await waitFor(() => {
       expect(screen.getAllByTestId('metric-toolbar')).toHaveLength(1);
     });
+
+    // Wait for the SearchQueryBuilderCombobox inside the re-expanded sidebar to
+    // finish initialising, otherwise its async state updates fire after the test
+    // ends and trigger an act() warning that jest-fail-on-console catches.
+    await waitFor(() => {
+      expect(
+        within(screen.getAllByTestId('metric-toolbar')[0]!).getByRole('combobox')
+      ).toBeInTheDocument();
+    });
   });
 });