fix(failure_store): make circuit state transitions atomic using Redis pipeline

[zerexei]

Fixes #2

Summary

This PR refactors FailureStore.set_state() to execute state updates and cooldown key management atomically using Redis transaction pipelines (pipeline(transaction=True)).

Changes

• Updated FailureStore.set_state() to wrap operations inside async with self.redis.pipeline(transaction=True) as pipe:.
• Updated test helper FakeRedis across unit, integration, and chaos test suites to support pipeline().
• Added unit test test_atomic_state_transition in tests/test_failure_store.py.

Verification

Ran uv run pytest — 50/50 tests passed.

[zerexei]

update docs if needed