Add initial test coverage using pytest and pytest-mock

[lbrealdev]

This PR introduces the initial test infrastructure for the project using pytest and pytest-mock.
It includes:

• Setup of test environment and dependencies.
• Unit tests for:
  • fetch_user: Validates the retrieval of the authenticated user's login.
  • create_repository: Covers both user and organization repository creation flows.
• Mocking of external API calls to GitHub using pytest-mock.

This PR is related to the following issue:

• Configure Python Tests #33