refactor: Extract common git repository preparation logic (#229)

[SergK]

Consolidate duplicated git repository setup code across chain handlers into a reusable PrepareGitRepository function and GitRepositoryContext struct.

Changes:

• Add GitRepositoryContext struct to hold git operation context
• Add PrepareGitRepository function for common git setup workflow
• Refactor PutDeployConfigs to use PrepareGitRepository
• Refactor PutGitLabCIConfig to use PrepareGitRepository
• Add comprehensive unit tests for PrepareGitRepository

This reduces code duplication and improves maintainability by centralizing GitServer retrieval, SSH credential extraction, repository cloning, and branch checkout logic.

Fixes #229

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which improves an existing feature or documentation)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Checklist:

• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Pull Request contains one commit. I squash my commits.