GitAuto: Add unit tests to services/github/branch_manager.py

[gitauto-for-dev]

Resolves #468

What is the feature

Add comprehensive unit tests for the services/github/branch_manager.py module to ensure its reliability and maintainability.

Where / How to implement the feature

• Create or update the test file tests/services/github/test_branch_manager.py to include unit tests covering all public methods and functionalities of the branch_manager.py module.
• Utilize pytest as the testing framework to maintain consistency with existing tests and leverage its powerful features.
• Mock external dependencies such as GitHub API calls using unittest.mock or similar libraries to isolate unit tests and ensure they run reliably without external factors.
• Ensure high test coverage by including various scenarios, including edge cases and error handling, to comprehensively validate the behavior of the branch_manager.py module.
• Follow best practices for writing clean, readable, and maintainable test code, adhering to the project's testing conventions and guidelines.

Anything the issuer needs to do

No action required.

Test these changes locally

git fetch origin
git checkout gitauto-wes/issue-468-20250108-201157
git pull origin gitauto-wes/issue-468-20250108-201157