* Fix DNS probe thread safety and avoid blocking engine sync
Refactor ProbeAvailability to prevent blocking the engine's sync mutex
during slow DNS probes. The probe now derives its context from the
server's own context (s.ctx) instead of accepting one from the caller,
and uses a mutex to ensure only one probe runs at a time — new calls
cancel the previous probe before starting. Also fixes a data race in
Stop() when accessing probeCancel without the probe mutex.
* Ensure DNS probe thread safety by locking critical sections
Add proper locking to prevent data races when accessing shared resources during DNS probe execution and Stop(). Update handlers snapshot logic to avoid conflicts with concurrent writers.
* Rename context and remove redundant cancellation
* Cancel first and lock
* Add locking to ensure thread safety when reactivating upstream servers
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