test: add comprehensive test suite - Closes #7

Added comprehensive test coverage for all tool modules:

Test Files Created:
- tests/conftest.py: Shared fixtures for all tests
- tests/test_issues.py: Complete coverage for issue tools
- tests/test_labels.py: Complete coverage for label tools
- tests/test_milestones.py: Complete coverage for milestone tools

Test Coverage:
- Tool definition validation (schema structure)
- Handler function routing
- Successful API response formatting
- Error handling (GiteaClientError)
- Required parameter validation
- Optional parameter handling
- Mock Gitea API responses

Configuration Updates:
- Added pytest-cov>=4.0.0 to dev dependencies
- Created run_tests.sh script for easy test execution

All tests use pytest-asyncio for async functions and mock the
GiteaClient to avoid real API calls. Tests validate tool schemas,
handler routing, response formatting, and error handling.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -33,6 +33,7 @@ dependencies = [
 dev = [
     "pytest>=7.0.0",
     "pytest-asyncio>=0.21.0",
+    "pytest-cov>=4.0.0",
 ]
 
 [project.scripts]