chore: Improve testing for atomic memory store

[jsonbailey]

Note

Strengthens test coverage around data store behavior, key handling, persistence, and concurrency.

• InMemoryFeatureStoreV2: Adds tests for initialized?, string/symbol key compatibility in get, filtering in all, full replacement via set_basis, incremental updates via apply_delta (incl. deletes and error handling), and thread-safety under concurrent reads/writes.
• DataStore::Store: New spec validates applying change sets (TRANSFER_FULL/TRANSFER_CHANGES/TRANSFER_NONE), DELETE handling, change set broadcasting, and commit semantics with/without persistent store (READ_WRITE vs READ_ONLY), including encoding, upsert usage, and failure propagation.
• FDv2 persistence: Extends tests to verify delta updates are persisted in READ_WRITE but not READ_ONLY, combined initializer+synchronizer persistence behavior, and presence/behavior of the data store status provider (with or without a persistent store).

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