fix: resolve async implementation bugs and race conditions

[27Bslash6]

Summary

• Fix async pool race conditions in Redis client with double-checked locking to prevent multiple pool creation
• Add proper async pool disconnect handling in reset_global_pool() when event loop is running
• Fix missing correlation ID cleanup in async wrapper finally block (line 1247)
• Update async operations to run in thread pool to avoid blocking event loop
• Add comprehensive async context leak regression tests
• Update documentation to reflect native async support with code examples

Changes

• Redis Client: Fixed race conditions in get_async_redis_client() with asyncio.Lock
• Pool Management: Proper disconnect handling in reset_global_pool()
• Async Wrapper: Added missing features.clear_correlation_id() in finally block
• Cache Handler: Modified async methods to run sync operations in thread pool
• Documentation: Updated comparison.md with async support examples and evidence
• Tests: Added extensive async context leak and pool lifecycle tests

Testing

• 50+ async tests validate full async support including L1/L2 caching, circuit breaker, and distributed locking
• Added concurrent async operation tests to verify no race conditions
• Added context leak regression tests for both success and failure paths

Resolves multiple async stability issues identified in production testing.

[codecov]

Codecov Report

❌ Patch coverage is 41.02564% with 23 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/cachekit/backends/redis/client.py	36.84%	12 Missing ⚠️
src/cachekit/decorators/wrapper.py	20.00%	8 Missing ⚠️
src/cachekit/cache_handler.py	70.00%	2 Missing and 1 partial ⚠️

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