feat: Add async big segment store manager and async Redis adapter

[jsonbailey]

What

The async big-segments layer for the async Python SDK.

• impl/async_big_segments.py — AsyncBigSegmentStoreManager (async twin of the sync manager: membership caching via ExpiringDict, status polling via AsyncRepeatingTask) + BigSegmentStoreStatusProviderImpl.
• impl/integrations/redis/async_redis_big_segment_store.py — async Redis-backed AsyncBigSegmentStore (redis.asyncio).
• integrations/__init__.py — new Redis.async_big_segment_store(...) factory (local import of the adapter, so redis stays optional).
• async_config.py — seeded with AsyncBigSegmentsConfig only (see below).
• Tests: testing/impl/test_async_big_segments.py (unit) + testing/integrations/test_async_redis.py (live-Redis integration, guarded by skip_database_tests).
• pyproject.toml — the published [redis] extra floor is unchanged (>=2.10.5); only the dev/test redis dependency is bumped to >=4.2.0 so CI can exercise redis.asyncio. Async Redis support requires redis>=4.2.0, surfaced via a README note and the adapter's guarded import (a clear error if redis.asyncio is unavailable) rather than by raising the consumer-facing floor. Raising the published floor is deferred pending a team decision before the async client ships.

Note on async_config.py

AsyncBigSegmentStoreManager needs AsyncBigSegmentsConfig, which lives in async_config.py. The full AsyncConfig isn't extractable yet (it imports AsyncHook/AsyncPlugin/AsyncEventProcessor, which land in later slices), so this PR seeds async_config.py with just the self-contained AsyncBigSegmentsConfig (it depends only on the already-merged AsyncBigSegmentStore). A later slice grows the file with AsyncConfig — mirroring how the sync SDK keeps BigSegmentsConfig alongside Config in config.py.

Why

Part of the async Python SDK (epic SDK-60). Depends on the async support classes (#451) and async interfaces/store (#457), both merged.

Validation

• uv run pytest ldclient/testing --ignore=ldclient/testing/integrations — 1033 passed, 2 skipped
• uv run pytest ldclient/testing/impl/test_async_big_segments.py — 14 passed
• uv run mypy ldclient — clean; isort / pycodestyle — clean
• Redis integration tests (test_async_redis.py) require a live Redis (run in CI); properly guarded by skip_database_tests.

SDK-2729 (epic SDK-60).

---

Note

Medium Risk
New experimental async evaluation path and Redis I/O affect segment targeting when adopted; sync behavior should be unchanged aside from the shared-helper refactor.

Overview
Adds the experimental async Big Segments stack for the async Python SDK: AsyncBigSegmentsConfig in async_config.py, AsyncBigSegmentStoreManager (membership cache, AsyncRepeatingTask status polling, inline poll before first membership read), and _AsyncRedisBigSegmentStore via new Redis.async_big_segment_store().

Sync code is refactored by moving BigSegmentStoreStatusProviderImpl, EMPTY_MEMBERSHIP, _hash_for_user_key, and is_stale into big_segments_common.py so sync and async managers share the same logic.

Dev dependency redis is bumped to >=4.2.0 for redis.asyncio in tests; the published [redis] extra floor is unchanged. Unit and live-Redis integration tests cover the new paths.

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