[abdullahjaved06]

Description

Adds a new documentation page (docs/customization/testing_plugins.md) explaining how plugin authors can write automated tests for their Commitizen plugins. It includes a complete example plugin and tests for version bumps, changelog extraction, message creation, and schema validation. Also links to it from python_class.md and adds it to the nav. Closes #937.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

The new page appears under Advanced Customization → Testing plugins and renders correctly. The example plugin and its tests are copy-pasteable and pass against the current Commitizen release.

Steps to Test This Pull Request

1. Run uv run poe doc and open the “Testing plugins” page; confirm it renders.
2. In a clean environment with commitizen and pytest, create the cz_example.py and tests/test_plugin.py files from the guide.
3. Run python -m pytest tests/test_plugin.py — all tests pass (I verified: 24 passed).

Additional Context