fix: Address potential race condition in FeatureStore update_availability

[jsonbailey]

Note

Improves robustness of store availability monitoring and minor logging tweaks.

• Refactors FeatureStoreClientWrapper.__update_availability to compute state changes and poller actions under __lock, then stop/start RepeatingTask outside the lock to avoid race/deadlock
• Adds explicit warning when the persistent store becomes unavailable; retains recovery log when it becomes available
• Updates FDv2 synchronizer loop error message from "primary synchronizer" to generic "synchronizer"

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