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

@@ -730,8 +730,14 @@ describe('MetricsTabContent (tracemetrics-ui-refresh)', () => {
 
       await userEvent.click(screen.getByRole('button', {name: 'Expand sidebar'}));
 
+      const metricToolbar = await waitFor(() => {
+        return screen.getByTestId('metric-toolbar');
+      });
+
+      // Wait for the SearchQueryBuilderCombobox inside the re-expanded sidebar to
+      // finish initialising, otherwise its async state updates fire after test end
       await waitFor(() => {
-        expect(screen.getAllByTestId('metric-toolbar')).toHaveLength(1);
+        expect(within(metricToolbar).getByRole('combobox')).toBeInTheDocument();
       });
     }
   );