Add unit tests for the basic functionality

## Add Unit Tests for Core Components

Add pytest-based unit tests for elastic-evals core logic including types, config, RAG metrics, statistics, and client components with VCR for API snapshots.

### Scope
- Test infrastructure with pytest fixtures and VCR setup
- Core logic tests: types, config, RAG metrics, statistics, comparisons
- Client tests: inference client and evaluators with mocked APIs