test: add pytest-mock, introduce fixtures & type hints (#290)

* Added pytest-mock to dev dependencies and pre-commit hooks
* Introduced InvalidGitHubTokenError for clearer token-validation failures
* Refactored tests:
  * Replaced ad-hoc mocks with reusable fixtures
  * Parametrised URL/branch matrices to cut duplication
  * Added type hints throughout
* New coverage:
  * validate_github_token (happy & error paths)
  * create_git_command / create_git_auth_header

.pre-commit-config.yaml

@@ -99,6 +99,7 @@ repos:
             "fastapi[standard]>=0.109.1",
             pydantic,
             pytest-asyncio,
+            pytest-mock,
             python-dotenv,
             slowapi,
             starlette>=0.40.0,
@@ -117,6 +118,7 @@ repos:
             "fastapi[standard]>=0.109.1",
             pydantic,
             pytest-asyncio,
+            pytest-mock,
             python-dotenv,
             slowapi,
             starlette>=0.40.0,