Skip LostNodeEvent when all search nodes are lost

What does this MR do and why?

Skip LostNodeEvent when all search nodes are lost

Prevents firing LostNodeEvent when all Zoekt search nodes are lost to avoid unnecessary event processing. The check now ensures that at least one online search node exists before triggering the lost node handling workflow.

References

Closes #577888

Screenshots or screen recordings

Not applicable - backend logic change only.

How to set up and validate locally

1. Set up Zoekt nodes in your development environment
2. Run the scheduling service task:

   Search::Zoekt::SchedulingService.execute!(:lost_nodes_check)

3. Verify that when all nodes are marked as lost, no LostNodeEvent is published
4. Run the test suite to validate the logic:

   bin/rspec ee/spec/services/search/zoekt/scheduling_service_spec.rb --example "lost_nodes_check"

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