[keelerm84]

fix: Prevent flag change listeners from firing for writes the store rejects

Summary

FeatureStore.upsert and FeatureStore.delete must now return a bool: True if the store applied the write, False if it rejected it because the stored version is equal or newer. This matches the async store contract.

• The data source update sink now updates dependencies and sends flag change events only when the store applied the write. Before, a rejected write still fired change events.
• InMemoryFeatureStore and CachingStoreWrapper return the result. CachingStoreWrapper derives it the same way the async wrapper does: upsert_internal must return the passed-in item on success. The FeatureStoreCore interface is unchanged; its docstring now states that requirement. The Redis, DynamoDB and Consul cores already meet it.
• Resolves the old TODOs in test_data_sink.py; those tests now assert that a rejected write sends no event.

The Redis suites pass locally. The DynamoDB and Consul suites run in CI only.

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Note

Overview
Breaking change: FeatureStore.upsert and FeatureStore.delete now return bool (True when the write is applied, False when it is rejected because the stored version is equal or newer), aligning the sync store contract with the async store.

DataSourceUpdateSinkImpl uses that result so dependency tracking and flag change listeners run only when the store actually changed. Stale or duplicate stream/poll updates no longer fire spurious change events.

Built-in paths implement the contract: InMemoryFeatureStore returns explicit success/failure; CachingStoreWrapper derives it from upsert_internal (success when the core returns the same object that was passed in). FDv1/FDv2 _FeatureStoreClientWrapper forwards the boolean. Interface docs clarify the core return-value rule and fix a typo in the delete docstring.

Tests assert return values on store operations and add coverage that a rejected upsert does not notify listeners; prior TODOs about duplicate deletes/upserts are resolved by expecting no second event when the store rejects the write.

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