Add git installation check for repository providers

[BillJr99]

Summary

Adds automatic git installation verification and fallback installation for repository providers. This ensures git is available when cloning and building repositories, with graceful handling for environments where it may not be pre-installed.

Key Changes

• New _ensure_git_installed() function in server.py: Checks if git is on PATH; if missing, attempts to install it via apt-get. Raises a descriptive error if installation fails or git remains unavailable after installation.
• Integration points: Called in both materialize_repository() (server.py) and clone_and_build() (frontend/app.py) before repository operations.
• Dockerfile update: Adds git to the base image build to ensure it's available in Docker environments, reducing reliance on runtime installation.
• Comprehensive test coverage: New TestEnsureGitInstalled class with three test cases covering success path, apt-get installation, and error handling.

Implementation Details

• Uses shutil.which("git") to detect git availability
• Attempts apt-get update followed by apt-get install -y --no-install-recommends git
• Includes validation that git is actually on PATH after installation
• Provides clear error messages distinguishing between missing git and failed installation
• Designed to run at most once per process (though not explicitly cached in this implementation)

https://claude.ai/code/session_01R2Vi6NE4wGxwv6rZ2HEXji