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>

run_tests.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# Test runner script for gitea-mcp-remote
+
+set -e
+
+echo "Running gitea-mcp-remote test suite..."
+echo
+
+# Install dev dependencies if not already installed
+if ! command -v pytest &> /dev/null; then
+    echo "Installing dev dependencies..."
+    pip install -e ".[dev]"
+    echo
+fi
+
+# Run tests with coverage
+echo "Running tests with coverage..."
+python -m pytest tests/ -v --cov=src/gitea_mcp --cov-report=term-missing --cov-report=html
+
+echo
+echo "Coverage report saved to htmlcov/index.html"