fix: Add context manager for clearer, safer locks

[jsonbailey]

Note

Introduces context-managed read/write locking and refactors SDK code to use it for clearer, safer concurrency.

• Adds ReadWriteLock.read()/write() context managers and updates all locking sites to with blocks (replacing manual lock()/unlock() and rlock()/runlock()) across ldclient/__init__.py, client.py, feature_store.py, data source/store status providers, FDv2 datasystem (wrapper, store, lifecycle), listeners, flag tracker, and integrations/test_data*
• Minor internal cleanups while preserving behavior (e.g., poller start/stop guarded by write locks, listener lists copied under read locks)

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[kinyoklion]

A thing of beauty.