Issue #3572830: Warn when configured system pages are unroutable

Sourav Paulrequested to merge
issue/drupal-3572830:3572830-add-a-warning into main

Add a runtime status-report requirement in SystemRequirements that checks system.site page.front/page.403/page.404 and reports a warning when any configured path cannot be routed.

  • Added getUnroutableSystemPages() to centralize routability checks.
  • Uses path.validator->getUrlIfValidWithoutAccessCheck() to avoid access-based false positives.
  • Adds a “Configured system pages” warning entry with affected config keys and a link to Site information.

Closes #3572830

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