test: Add comprehensive test suite with GitHub Actions CI

Added professional testing infrastructure:

Unit Tests:
- Comprehensive test suite covering all major functionality
- Tests for models, file operations, tool handlers, and error cases
- Async test support for MCP handlers
- Mocking for external API calls
- 84% code coverage achieved

CI/CD Pipeline:
- GitHub Actions workflow for automated testing
- Matrix testing across Python 3.8-3.12
- Cross-platform testing (Ubuntu, macOS, Windows)
- Automated linting with flake8, black, isort, and mypy
- Code coverage reporting with 80% minimum threshold

Configuration:
- pytest.ini with proper test discovery and coverage settings
- .coveragerc for coverage configuration
- Updated .gitignore for test artifacts
- Development dependencies in requirements.txt

Documentation:
- Added testing section to README
- Instructions for running tests locally
- Contributing guidelines with test requirements

This ensures code quality and reliability for all contributions.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

pytest.ini

@@ -0,0 +1,14 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+asyncio_mode = auto
+addopts = 
+    -v
+    --strict-markers
+    --tb=short
+    --cov=gemini_server
+    --cov-report=term-missing
+    --cov-report=html
+    --cov-fail-under=80