chore: Ensure memory store operations are atomic

[keelerm84]

Note

Introduce ReadOnlyStore and refactor FDv2 store/application logic to use an in-memory read-only active store with set_basis/apply_delta, persisting deltas via upserts (including tombstones); update client, interfaces, protocol, tests, and bump launchdarkly-eventsource.

• Data system/store (FDv2):
  • Add in-memory InMemoryFeatureStore implementing ReadOnlyStore with set_basis and apply_delta.
  • Refactor Store to:
    • Convert Change lists to collections; on TRANSFER_FULL call set_basis, on TRANSFER_CHANGES call apply_delta.
    • Persist deltas via upsert (including DELETE as tombstone objects) when writable; remove direct delete calls.
    • Track selector, dependency updates, and emit flag change events from computed affected items.
  • Adjust FeatureStoreClientWrapper/persistence wiring; get_active_store now returns ReadOnlyStore.
• Client/FDv1/FDv2:
  • Expose ReadOnlyStore via DataSystem.store; client/evaluator now read from ReadOnlyStore.
  • Wire diagnostic accumulator; minor logging tweaks.
• Interfaces:
  • Add ReadOnlyStore protocol and update DataSystem/implementations to return it.
• Protocol:
  • Narrow Change.object type to Optional[dict].
• Dependency tracking:
  • Ignore items marked deleted when computing dependencies.
• Tests:
  • Update persistence tests to expect upserts for deletes and new collection typings.
• Dependencies:
  • Bump launchdarkly-eventsource to >=1.4.0,<2.0.0.

^{Written by Cursor Bugbot for commit 8bcddb1. This will update automatically on new commits. Configure here.}

[kinyoklion]

Is this intended to be in this PR? Or rebase problem?

[keelerm84]

This should have been in a previously merged PR. Not sure how that was missed.