feat: Add async feature store interface and in-memory implementation

[jsonbailey]

BEGIN_COMMIT_OVERRIDE
chore: Add async feature store interface and in-memory implementation
END_COMMIT_OVERRIDE

Stacked on #451 (async support classes). Review/merge that first; this PR's diff is against that branch.

Tracked internally: SDK-2671

What

The async data-store foundation the async SDK builds on.

• interfaces.py — async interfaces: AsyncFeatureStore, AsyncReadOnlyStore (async get/all; the async analog of ReadOnlyStore), AsyncDataSourceUpdateSink, AsyncBigSegmentStore, AsyncInitializer, AsyncSynchronizer. Pure additions alongside the existing sync interfaces.
• async_feature_store.py — AsyncInMemoryFeatureStore. Holds no lock: every method reads/mutates its dict without awaiting in between, so it runs to completion without the event loop switching coroutines (the sync twin needs a ReadWriteLock because threads can preempt each other).
• testing/ — async test utilities: mock_async_components.py, async_feature_store_test_base.py, and test_async_in_memory_feature_store.py.

Why

Foundation for the async Python SDK (epic SDK-60). Subsequent slices (async evaluator, big segments, data sources, data systems, client) depend on these interfaces and the in-memory store.

Validation

• uv run pytest ldclient/testing/test_async_in_memory_feature_store.py — 17 passed
• uv run mypy ldclient — clean
• isort / pycodestyle — clean

Epic SDK-60.

---

Note

Low Risk
Additive experimental APIs and in-memory store with no changes to existing sync behavior; risk is mainly API churn before production readiness.

Overview
Adds the async data-store foundation for the experimental async Python SDK: new async counterparts to the sync flag-data plumbing, plus a default in-memory implementation and shared tests.

interfaces.py gains async APIs alongside existing sync types: AsyncReadOnlyStore, AsyncFeatureStore (versioned init/upsert/delete, upsert returns whether an update applied), AsyncDataSourceUpdateSink, AsyncBigSegmentStore, and FDv2-style AsyncInitializer / AsyncSynchronizer (async fetch and async-generator sync). All are marked experimental.

async_feature_store.py introduces AsyncInMemoryFeatureStore, mirroring sync in-memory versioning and tombstone delete semantics but without a ReadWriteLock—methods do not await between reads/writes, so dict updates run atomically on the event loop.

testing/ adds AsyncFeatureStoreTestBase (async port of the sync store contract tests), MockAsyncFeatureStore / MockAsyncEventProcessor / MockAsyncUpdateProcessor, and test_async_in_memory_feature_store.py wiring the in-memory store through that suite.

^{Reviewed by Cursor Bugbot for commit 9966c03. Bugbot is set up for automated code reviews on this repo. Configure here.}