chore: Flush in memory store on persistent store recovery

[keelerm84]

Note

Flushes in-memory data to the persistent store on recovery with stale data, switches client evaluations to use the data system store, tightens polling selector handling, and adds thorough persistence tests.

• Data system (FDv2):
  • Outage recovery: Add _persistent_store_outage_recovery to flush via Store.commit() when DataStoreStatus(available=True, stale=True); register listener; set status with DataStoreStatus(available, True) on availability.
  • Store.commit: Encode FEATURES/SEGMENTS via VersionedDataKind.encode before calling persistent init().
• Client:
  • Use _data_system.store for evaluator sources, initialization checks, and all_flags_state() reads.
• Polling:
  • Only send selector query param when selector.is_defined().
• Protocol/typing:
  • Fix Synchronizer.sync return type to Generator["Update", None, None].
• Tests:
  • Add tests for outage recovery flush behavior, commit encoding, commit without persistent store, and error handling in commit.

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[kinyoklion]

So much larger a number.

[keelerm84]

It is, but also we were always allowing it to float later. We are actually pinning it behind because it's breaking in older versions now.