fix flaky test: alert details error page timeout (elastic#260302)

## Summary

Fixes the flaky test reported in elastic#250046.

**Root cause:** \`toBeVisible()\` would start checking while the
component was in its loading state (spinner), missing the brief initial
render of the error panel and timing out before the async API call for
the non-existent alert completed.

**Fix:** Wrap navigate+assert in \`.toPass({ timeout: 30_000, intervals:
[2_000] })\`. I also fixed a data-test-subj and a `goto` method that
changed during Rules page unification

**Test type:** Scout (Playwright)
**Test file:**
\`x-pack/solutions/observability/plugins/observability/test/scout/ui/parallel_tests/alert_details_page.spec.ts\`
**Config:**
\`x-pack/solutions/observability/plugins/observability/test/scout/ui/parallel.playwright.config.ts\`

## Validation

- [ ] CI flaky test runner (50 + 150 runs) 

## Checklist

- [x] Flaky Test Runner was used on any tests changed

Author: miguelmartin-elastic

x-pack/solutions/observability/plugins/observability/test/scout/ui/fixtures/page_objects/alert_page.ts

@@ -30,7 +30,7 @@ export class AlertPage {
 
     await expandAlertButtons[0].click();
 
-    const alertDetailsLink = this.page.testSubj.locator('alertsFlyoutAlertDetailsButton');
+    const alertDetailsLink = this.page.testSubj.locator('alertFlyoutAlertDetailsButton');
     await expect(alertDetailsLink).toBeVisible();
     await alertDetailsLink.click();
 

x-pack/solutions/observability/plugins/observability/test/scout/ui/fixtures/page_objects/rules_page.ts

@@ -24,7 +24,7 @@ export class RulesPage {
    * Navigates to the rules list page (Observability)
    */
   async goto(ruleId: string = '') {
-    await this.page.gotoApp(`rules/${ruleId}`);
+    await this.page.gotoApp(ruleId ? `rules/rule/${ruleId}` : 'rules');
     if (!ruleId) {
       await this.page.testSubj.waitForSelector(RULES_SETTINGS_TEST_SUBJECTS.RULE_PAGE_TAB, {
         timeout: BIGGER_TIMEOUT,

x-pack/solutions/observability/plugins/observability/test/scout/ui/parallel_tests/alert_details_page.spec.ts

@@ -10,8 +10,7 @@ import { expect } from '@kbn/scout-oblt/ui';
 import { test } from '../fixtures';
 import { GENERATED_METRICS } from '../fixtures/constants';
 
-// Failing: See https://github.com/elastic/kibana/issues/250046
-test.describe.skip(
+test.describe(
   'Alert Details Page',
   { tag: [...tags.stateful.classic, ...tags.serverless.observability.complete] },
   () => {
@@ -58,13 +57,16 @@ test.describe.skip(
       })) as { data: { id: string } };
       ruleId = createdRule.data.id;
     });
+
     test.beforeEach(async ({ browserAuth }) => {
       await browserAuth.loginAsAdmin();
     });
 
     test('should show an error when the alert does not exist', async ({ page, pageObjects }) => {
-      await pageObjects.alertPage.goto('non-existent-alert-id');
-      await expect(page.testSubj.locator('alertDetailsError')).toBeVisible();
+      await expect(async () => {
+        await pageObjects.alertPage.goto('non-existent-alert-id');
+        await expect(page.testSubj.locator('alertDetailsError')).toBeVisible();
+      }).toPass({ timeout: 30_000, intervals: [2_000] });
     });
 
     test('should show a tabbed view', async ({ page, pageObjects }) => {