Fix AlternatorLiveNodes thread lifecycle

[dkropachev]

Summary

• Add proper shutdown mechanism to AlternatorLiveNodes background thread
• Fix infinite loop without shutdown coordination
• Add graceful termination methods

Changes

• Add shutdownRequested AtomicBoolean flag for coordinated shutdown
• Add public shutdown() method to initiate thread termination
• Add isRunning() method to check thread status
• Update run() loop to check shutdown flag instead of infinite loop
• Restore interrupted status when thread receives InterruptedException

Test plan

• Verify compilation passes
• Test thread can be shut down gracefully via shutdown() method
• Verify thread terminates when interrupt is called
• Check that running status is tracked correctly