Refactor: Extract common git repository preparation logic into reusable function

[SergK]

Is your feature request related to a problem? Please describe.

The codebase has duplicated git repository preparation logic across multiple chain handlers (put_deploy_configs, put_gitlab_ci_config). Each handler independently retrieves GitServer resources, extracts SSH credentials, clones repositories, and checks out branches, leading to code duplication and increased maintenance burden.

Describe the solution you'd like

Extract the common git repository preparation workflow into a reusable function:

• Create a GitRepositoryContext struct to hold necessary git operation context (GitServer, Secret, SSH keys, URLs, working directory)
• Implement a PrepareGitRepository function that handles:
  • GitServer resource and Secret retrieval
  • SSH credential extraction
  • Repository cloning (if needed)
  • Default branch checkout
• Refactor existing chain handlers to use this common function
• Add comprehensive unit tests for the new functionality

Describe alternatives you've considered

• Keep the current duplicated code - rejected due to maintainability concerns
• Use a builder pattern - overly complex for this straightforward use case
• Create a separate service struct - adds unnecessary abstraction

Additional context

This refactoring improves code maintainability by centralizing git operations setup logic and makes it easier to add new chain handlers that require similar git operations. The change affects put_deploy_configs.go and put_gitlab_ci_config.go, reducing ~50 lines of duplicated code per handler.