feat: implement MCP server core and authentication

Implemented MCP server core infrastructure with authentication and HTTP client:

- Created auth.py for API token management
  - Loads GITEA_API_URL and GITEA_API_TOKEN from environment
  - Uses python-dotenv for .env file support
  - Validates required configuration on initialization
  - Provides authentication headers for API requests

- Created client.py with base HTTP client
  - GiteaClient class using httpx AsyncClient
  - Async HTTP methods: get(), post(), patch(), delete()
  - Comprehensive error handling for HTTP status codes
  - Custom exception hierarchy for different error types
  - Configurable timeout (default 30s)

- Updated server.py with MCP server setup
  - Initialized MCP server with StdioServerTransport
  - Integrated AuthConfig and GiteaClient
  - Registered placeholder tool handlers (list_repositories, create_issue, create_pull_request)
  - Added CLI with --help and --version options
  - Proper error handling for configuration failures

- Updated pyproject.toml
  - Added console script entry point: gitea-mcp

- Created comprehensive unit tests
  - test_auth.py: Tests for AuthConfig validation and headers
  - test_client.py: Tests for GiteaClient initialization and error handling

All acceptance criteria met:
- MCP server initializes with StdioServerTransport
- Authentication loads from environment variables
- Base HTTP client with auth headers implemented
- Error handling for API connection failures
- Server reports available tools (placeholders for future issues)

Closes #2

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

tests/test_client.py

@@ -0,0 +1,93 @@
+"""Tests for Gitea API client."""
+
+import pytest
+import httpx
+from unittest.mock import AsyncMock, MagicMock
+from gitea_mcp.auth import AuthConfig
+from gitea_mcp.client import (
+    GiteaClient,
+    GiteaClientError,
+    GiteaAuthError,
+    GiteaNotFoundError,
+    GiteaServerError,
+)
+
+
+@pytest.fixture
+def mock_config(monkeypatch):
+    """Create mock authentication config."""
+    monkeypatch.setenv("GITEA_API_URL", "http://gitea.example.com/api/v1")
+    monkeypatch.setenv("GITEA_API_TOKEN", "test_token_123")
+    return AuthConfig()
+
+
+@pytest.mark.asyncio
+async def test_client_initialization(mock_config):
+    """Test client initialization."""
+    client = GiteaClient(mock_config, timeout=10.0)
+    assert client.config == mock_config
+    assert client.timeout == 10.0
+
+
+@pytest.mark.asyncio
+async def test_client_context_manager(mock_config):
+    """Test client as async context manager."""
+    async with GiteaClient(mock_config) as client:
+        assert client._client is not None
+        assert isinstance(client._client, httpx.AsyncClient)
+
+
+@pytest.mark.asyncio
+async def test_client_without_context_manager_raises_error(mock_config):
+    """Test that using client without context manager raises error."""
+    client = GiteaClient(mock_config)
+
+    with pytest.raises(GiteaClientError, match="not initialized"):
+        await client.get("/test")
+
+
+@pytest.mark.asyncio
+async def test_handle_error_401(mock_config):
+    """Test handling 401 authentication error."""
+    response = MagicMock()
+    response.status_code = 401
+
+    async with GiteaClient(mock_config) as client:
+        with pytest.raises(GiteaAuthError, match="Authentication failed"):
+            client._handle_error(response)
+
+
+@pytest.mark.asyncio
+async def test_handle_error_403(mock_config):
+    """Test handling 403 forbidden error."""
+    response = MagicMock()
+    response.status_code = 403
+
+    async with GiteaClient(mock_config) as client:
+        with pytest.raises(GiteaAuthError, match="Access forbidden"):
+            client._handle_error(response)
+
+
+@pytest.mark.asyncio
+async def test_handle_error_404(mock_config):
+    """Test handling 404 not found error."""
+    response = MagicMock()
+    response.status_code = 404
+    response.request = MagicMock()
+    response.request.url = "http://gitea.example.com/api/v1/test"
+
+    async with GiteaClient(mock_config) as client:
+        with pytest.raises(GiteaNotFoundError, match="not found"):
+            client._handle_error(response)
+
+
+@pytest.mark.asyncio
+async def test_handle_error_500(mock_config):
+    """Test handling 500 server error."""
+    response = MagicMock()
+    response.status_code = 500
+    response.text = "Internal Server Error"
+
+    async with GiteaClient(mock_config) as client:
+        with pytest.raises(GiteaServerError, match="server error"):
+            client._handle_error(response)