Enhance Testing Infrastructure and Documentation

[teocns]

Description

This PR improves the testing infrastructure and documentation by:

1. Moving detailed testing documentation from CONTRIBUTING.md to a dedicated tests/README.md
2. Adding comprehensive VCR.py configuration for HTTP interaction testing
3. Implementing test fixtures for LLM event spying
4. Adding proper test organization and best practices documentation

Key Changes

• Created dedicated testing documentation in tests/README.md
• Added VCR.py configuration in conftest.py for recording/replaying HTTP interactions
• Implemented session-wide VCR stats tracking
• Added filtering for sensitive API headers and credentials
• Updated OpenAI integration tests with VCR markers (see Add OpenAI Integration Test (Sync/Async + Streaming) #598 )
• Added new test fixtures for LLM providers
• Added Constantin Teodorescu to contributors list

Testing

The changes have been tested by:

• Running the full test suite
• Verifying VCR cassette recording/playback
• Testing OpenAI integration with the new infrastructure

Documentation

• Moved testing documentation to dedicated tests/README.md
• Added VCR.py usage instructions
• Updated test organization guidelines
• Added best practices for writing tests