[ci:fix] Prevent circular import in changelog bot Commitizen validation

[pushpitkamboj]

Issue Description

• The changelog bot can fail at runtime while validating generated commit messages against the OpenWISP Commitizen rules.
• The failure happens in generate_changelog.py when it imports OpenWispCommitizen from openwisp_utils.releaser.commitizen.
• That module imports commitizen.cz.base, which triggers Commitizen plugin discovery.
• Since openwisp-utils already registers OpenWispCommitizen as a commitizen.plugin entry point, Commitizen tries to import the same module again during initialization.
• This leads to a circular import and raises an error similar to:
  • AttributeError: partially initialized module 'openwisp_utils.releaser.commitizen' has no attribute 'OpenWispCommitizen'
• As a result, the GitHub Action for changelog generation can fail before posting the generated suggestion.
• see https://github.com/openwisp/openwisp-controller/actions/runs/25575877428/job/75082832879

Expected behavior

• The changelog bot should be able to reuse the OpenWISP Commitizen validation rules without triggering Commitizen plugin auto-discovery.

Proposed direction

• Avoid normal importing of openwisp_utils.releaser.commitizen inside the changelog action.
• Load only the plugin class and validation logic in a way that does not initialize Commitizen’s plugin registry.

[openwisp-companion]

This issue has been automatically assigned to @pushpitkamboj who opened PR #670 to address it. 🎯