Implement reusable cache store abstraction with in-memory and Redis
backends as foundation for prompt caching feature (PR1 of progressive delivery).
- Add CacheStore protocol defining cache interface
- Implement MemoryCacheStore with LRU, LFU, and TTL-only eviction policies
- Implement RedisCacheStore with connection pooling and retry logic
- Add CircuitBreaker for cache backend failure protection
- Include comprehensive unit tests (55 tests, >80% coverage)
- Add dependencies: cachetools>=5.5.0, redis>=5.2.0
This abstraction enables flexible caching implementations for the prompt
caching middleware without coupling to specific storage backends.
Signed-by: William Caban <willliam.caban@gmail.com>
