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>

tests/conftest.py

@@ -0,0 +1,103 @@
+"""Shared pytest fixtures for Gitea MCP tests."""
+
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+from gitea_mcp.auth import AuthConfig
+from gitea_mcp.client import GiteaClient
+
+
+@pytest.fixture
+def mock_config(monkeypatch):
+    """Create mock authentication config.
+
+    This fixture sets up test environment variables and returns
+    a configured AuthConfig instance for testing.
+    """
+    monkeypatch.setenv("GITEA_API_URL", "http://gitea.example.com/api/v1")
+    monkeypatch.setenv("GITEA_API_TOKEN", "test_token_123")
+    return AuthConfig()
+
+
+@pytest.fixture
+def mock_client(mock_config):
+    """Create a mock GiteaClient instance.
+
+    Returns a GiteaClient with mocked HTTP methods that don't make
+    real API calls. Use this for testing tool handlers.
+    """
+    client = GiteaClient(mock_config, timeout=10.0)
+
+    # Mock the internal HTTP client methods
+    client.get = AsyncMock()
+    client.post = AsyncMock()
+    client.patch = AsyncMock()
+    client.delete = AsyncMock()
+
+    # Mock context manager
+    client.__aenter__ = AsyncMock(return_value=client)
+    client.__aexit__ = AsyncMock(return_value=None)
+
+    return client
+
+
+@pytest.fixture
+def sample_issue():
+    """Sample issue data for testing.
+
+    Returns a dict representing a typical Gitea issue response.
+    """
+    return {
+        "id": 1,
+        "number": 42,
+        "title": "Test Issue",
+        "body": "This is a test issue",
+        "state": "open",
+        "created_at": "2024-01-15T10:00:00Z",
+        "updated_at": "2024-01-15T12:00:00Z",
+        "html_url": "http://gitea.example.com/test/repo/issues/42",
+        "labels": [
+            {"id": 1, "name": "bug", "color": "ff0000"},
+            {"id": 2, "name": "priority-high", "color": "ff9900"},
+        ],
+        "milestone": {
+            "id": 10,
+            "title": "v1.0",
+            "state": "open",
+        },
+        "assignees": [
+            {"id": 100, "login": "testuser"},
+        ],
+    }
+
+
+@pytest.fixture
+def sample_label():
+    """Sample label data for testing.
+
+    Returns a dict representing a typical Gitea label response.
+    """
+    return {
+        "id": 1,
+        "name": "bug",
+        "color": "ff0000",
+        "description": "Something isn't working",
+    }
+
+
+@pytest.fixture
+def sample_milestone():
+    """Sample milestone data for testing.
+
+    Returns a dict representing a typical Gitea milestone response.
+    """
+    return {
+        "id": 10,
+        "title": "v1.0",
+        "description": "First major release",
+        "state": "open",
+        "open_issues": 5,
+        "closed_issues": 15,
+        "created_at": "2024-01-01T00:00:00Z",
+        "updated_at": "2024-01-15T12:00:00Z",
+        "due_on": "2024-12-31T23:59:59Z",
+    }