test: add gptpr/test_gh.py

[alissonperez]

This PR adds a Pytest suite for the GitHub integration in the gptpr module.

Specifically:

• Adds gptpr/test_gh.py, covering:
  • successful PR creation, mocking the GitHub repository
  • cancellation flow when the user declines confirmation

These tests use pytest and InquirerPy, providing confidence that the create_pr function behaves correctly in both success and cancellation scenarios.

Next steps:

• Expand tests to cover edge cases and error handling
• Integrate CI to run these tests automatically