fix(test): stabilize flaky allMonitors select all query results test (#111901)

The test creates 20 filtered detectors and sets `X-Hits: 50`, then
asserts on "20 monitors on this page selected" and "Select all 50
monitors that match this search query". With that many fixtures and
complex rendering, the test can time out under CI load.

The fix reduces the fixture count from 20 to 3 and `X-Hits` from 50 to
10. That keeps the test's intent (verify select-all-across-pages
behavior) but with less intensive work.

Fixes ENG-7204

Made with [Cursor](https://cursor.com)

Co-authored-by: Claude Sonnet 4 <noreply@anthropic.com>

Author: 2 people

static/app/views/detectors/list/allMonitors.spec.tsx

@@ -505,7 +505,7 @@ describe('DetectorsList', () => {
 
         const testUser = UserFixture({id: '2', email: 'test@example.com'});
         // Mock the filtered search results - this will be used when search is applied
-        const filteredDetectors = Array.from({length: 20}, (_, i) =>
+        const filteredDetectors = Array.from({length: 3}, (_, i) =>
           MetricDetectorFixture({
             id: `filtered-${i}`,
             name: `Assigned Detector ${i + 1}`,
@@ -517,7 +517,7 @@ describe('DetectorsList', () => {
           url: '/organizations/org-slug/detectors/',
           body: filteredDetectors,
           headers: {
-            'X-Hits': '50',
+            'X-Hits': '10',
           },
           match: [
             MockApiClient.matchQuery({
@@ -548,9 +548,9 @@ describe('DetectorsList', () => {
         await userEvent.click(masterCheckbox);
 
         // Should show alert with option to select all query results
-        expect(screen.getByText(/20 monitors on this page selected/)).toBeInTheDocument();
+        expect(screen.getByText(/3 monitors on this page selected/)).toBeInTheDocument();
         const selectAllForQuery = screen.getByRole('button', {
-          name: /Select all 50 monitors that match this search query/,
+          name: /Select all 10 monitors that match this search query/,
         });
         await userEvent.click(selectAllForQuery);