ROX-13227: Add UI reachability check to CNAME manager

[ludydoo]

Summary

• Add UI reachability check to verify Central UI is accessible from internet before marking routes as created
• Implement mockable UIReachabilityChecker interface for testability
• Check reachability after CNAME records are in sync (INSYNC status)
• Support both fleet-manager managed CNAME records and external-dns managed scenarios

Changes

• Added UIReachabilityChecker interface with HTTP-based implementation
• Integrated reachability check in CentralRoutesCNAMEManager.Reconcile() method
• Added comprehensive test coverage with mock implementations
• Only mark routes as created when UI is actually reachable from internet

Testing

• Added unit tests covering various scenarios:
  • UI reachable after CNAME sync
  • UI not reachable after CNAME sync
  • Reachability check failures
  • External DNS managed scenarios
  • Empty UI host handling

Related

• Complements the fleetshard-sync UI reachability implementation in PR Bump k8s.io/kubernetes from 1.32.6 to 1.32.7 #2422

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ludydoo

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[ludydoo]

/retest

[openshift-ci]

@ludydoo: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e	f2f7ae0	link	true	/test e2e

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