Add comprehensive tests for CachePoolAdapter and SamplingClient

- Introduce detailed unit tests for `CachePoolAdapter`: pending responses, message handling, and cache interactions.
- Expand `SamplingClient` tests to cover logging scenarios, edge cases, and response handling.
- Ensure exceptions, invalid data, and client-state transitions are tested for robustness.
- Include validation of debug logs to confirm proper diagnostic information is captured.

Author: klapaudius