Remove implicit assumption about git from test_git_directory_exists

[Dev-iL]

While testing 1.90.0rc0 a test failed:

FAILED tests/cli/test_logic.py::test_git_directory_exists - OSError: fatal: not a git repository (or any of the parent directories): .git

This is because the test_git_directory_exists() integration-style test currently depends on the checkout being a real Git worktree. This PR updates the test to unit-test get_git_base_directory() by mocking subprocess.run, which makes it stable under uv run tarball/sdist environments.

How I tested this

Test was failing, now it passes.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.