Add unit tests for API client and log client lazy initialization

- Introduced comprehensive tests for the `Session` struct, covering lazy initialization of API and log clients using both JWT and OAuth.
- Implemented tests for error handling scenarios, ensuring proper responses for uninitialized token refreshers and invalid API URLs.
- Added concurrent access tests to verify thread safety during client initialization.
- Enhanced overall test coverage for the session management logic, improving reliability and maintainability of the codebase.

Author: freeznet