test: Fix Service Dashboards Scout wait (strict progressbar locator) (elastic#267796)

## Summary

- Replaced `getByRole('progressbar')` in
`DashboardsTab.waitForDashboardsToLoad` with a wait for
`apmMainTemplateServiceAgentLoader` to be hidden. The generic
progressbar role matched multiple concurrent EUI loaders (chrome,
environment filter, service title, etc.) and caused Playwright **strict
mode** failures.
- Extended `waitForTabLoad` on the dashboards tab to wait for the same
main template loader after the unified search bar is visible, matching
`MetricsTab` / `ServiceDetailsPage.goToPage` and stabilizing navigation
before the unlink test.

## Testing

- Ran `service_dashboards.spec.ts` locally — passed. Expect serverless
observability Scout (`local-serverless-observability_complete`) to
validate after CI merges.

Cc flaky Scout/serverless monitoring: worth confirming stability once
merged.

Closes elastic#256350


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Co-authored-by: Cursor <cursoragent@cursor.com>

Author: jennypavlova

x-pack/solutions/observability/plugins/apm/test/scout/ui/fixtures/page_objects/service_details/dashboards_tab.ts

@@ -30,10 +30,15 @@ export class DashboardsTab extends ServiceDetailsTab {
     await this.page
       .getByTestId('apmUnifiedSearchBar')
       .waitFor({ state: 'visible', timeout: EXTENDED_TIMEOUT });
+    await this.page
+      .getByTestId('apmMainTemplateServiceAgentLoader')
+      .waitFor({ state: 'hidden', timeout: EXTENDED_TIMEOUT });
   }
 
   public async waitForDashboardsToLoad(): Promise<void> {
-    await expect(this.page.getByRole('progressbar')).toBeHidden({ timeout: EXTENDED_TIMEOUT });
+    await this.page
+      .getByTestId('apmMainTemplateServiceAgentLoader')
+      .waitFor({ state: 'hidden', timeout: EXTENDED_TIMEOUT });
     await this.addServiceDashboardButton.waitFor({ state: 'visible', timeout: EXTENDED_TIMEOUT });
   }