`pytest-recording` docs in `CONTRIBUTING.md`

[jamesbraza]

Fixes #409

Add documentation for pytest-recording and VCR cassettes in CONTRIBUTING.md.

• Add a section for pytest-recording plugin.
• Link to pytest-recording documentation.
• Provide a brief summary on how to record a new VCR using uv run pytest tests/desired_test_module.py.
• Link to the config in tests/conftest.py.
• Document header removals from responses.
• Add relative links for tests/cassettes and tests/conftest.py.
• Add a comment about keeping the cassettes less than 1 MB in size.

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