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>

.coveragerc

@@ -0,0 +1,24 @@
+[run]
+source = gemini_server
+omit = 
+    */tests/*
+    */venv/*
+    */__pycache__/*
+    */site-packages/*
+
+[report]
+exclude_lines =
+    pragma: no cover
+    def __repr__
+    if self.debug:
+    if settings.DEBUG
+    raise AssertionError
+    raise NotImplementedError
+    if 0:
+    if __name__ == .__main__.:
+    if TYPE_CHECKING:
+    class .*\bProtocol\):
+    @(abc\.)?abstractmethod
+
+[html]
+directory = htmlcov