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 — bumps redis to >=4.2.0 (the redis.asyncio API requires redis-py 4.2+).

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).

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Note

Medium Risk
Sync big-segment code moved to shared helpers (behavior should be unchanged), and the redis optional dependency minimum increases; the async path is experimental but will affect segment membership when wired into the async client.

Overview
Adds the experimental async Big Segments stack: AsyncBigSegmentsConfig, AsyncBigSegmentStoreManager (membership cache, background status polling via AsyncRepeatingTask, and an inline poll on the first membership lookup if the poller has not run yet), plus Redis.async_big_segment_store() backed by redis.asyncio.

Sync BigSegmentStoreManager is refactored to share big_segments_common (EMPTY_MEMBERSHIP, user-key hashing, staleness, and BigSegmentStoreStatusProviderImpl) with the async manager so behavior stays aligned without duplicating logic.

pyproject.toml raises the optional/dev redis floor to >=4.2.0 for redis.asyncio. Unit and live-Redis integration tests cover the new manager and adapter.

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