feat(llm-gateway): implement LLM Gateway MCP Server (#56)

Implements complete LLM Gateway MCP Server with:
- FastMCP server with 4 tools: chat_completion, list_models, get_usage, count_tokens
- LiteLLM Router with multi-provider failover chains
- Circuit breaker pattern for fault tolerance
- Redis-based cost tracking per project/agent
- Comprehensive test suite (209 tests, 92% coverage)

Model groups defined per ADR-004:
- reasoning: claude-opus-4 → gpt-4.1 → gemini-2.5-pro
- code: claude-sonnet-4 → gpt-4.1 → deepseek-coder
- fast: claude-haiku → gpt-4.1-mini → gemini-2.0-flash

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

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

mcp-servers/llm-gateway/tests/conftest.py

@@ -0,0 +1,204 @@
+"""
+Pytest fixtures for LLM Gateway tests.
+"""
+
+import os
+import sys
+from collections.abc import AsyncIterator, Iterator
+from typing import Any
+from unittest.mock import MagicMock, patch
+
+import fakeredis.aioredis
+import pytest
+
+# Add parent directory to path for imports
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from config import Settings
+from cost_tracking import CostTracker, reset_cost_tracker
+from failover import CircuitBreakerRegistry, reset_circuit_registry
+from providers import LLMProvider, reset_provider
+from routing import ModelRouter, reset_model_router
+
+
+@pytest.fixture
+def test_settings() -> Settings:
+    """Create test settings with mock API keys."""
+    return Settings(
+        host="127.0.0.1",
+        port=8001,
+        debug=True,
+        redis_url="redis://localhost:6379/0",
+        anthropic_api_key="test-anthropic-key",
+        openai_api_key="test-openai-key",
+        google_api_key="test-google-key",
+        litellm_timeout=30,
+        litellm_max_retries=2,
+        litellm_cache_enabled=False,
+        cost_tracking_enabled=True,
+        circuit_failure_threshold=3,
+        circuit_recovery_timeout=10,
+    )
+
+
+@pytest.fixture
+def settings_no_providers() -> Settings:
+    """Create settings with no providers configured."""
+    return Settings(
+        host="127.0.0.1",
+        port=8001,
+        debug=False,
+        redis_url="redis://localhost:6379/0",
+        anthropic_api_key=None,
+        openai_api_key=None,
+        google_api_key=None,
+        alibaba_api_key=None,
+        deepseek_api_key=None,
+    )
+
+
+@pytest.fixture
+def fake_redis() -> fakeredis.aioredis.FakeRedis:
+    """Create a fake Redis instance for testing."""
+    return fakeredis.aioredis.FakeRedis(decode_responses=True)
+
+
+@pytest.fixture
+async def cost_tracker(
+    fake_redis: fakeredis.aioredis.FakeRedis,
+    test_settings: Settings,
+) -> AsyncIterator[CostTracker]:
+    """Create a cost tracker with fake Redis."""
+    reset_cost_tracker()
+    tracker = CostTracker(redis_client=fake_redis, settings=test_settings)
+    yield tracker
+    await tracker.close()
+    reset_cost_tracker()
+
+
+@pytest.fixture
+def circuit_registry(test_settings: Settings) -> Iterator[CircuitBreakerRegistry]:
+    """Create a circuit breaker registry for testing."""
+    reset_circuit_registry()
+    registry = CircuitBreakerRegistry(settings=test_settings)
+    yield registry
+    reset_circuit_registry()
+
+
+@pytest.fixture
+def model_router(
+    test_settings: Settings,
+    circuit_registry: CircuitBreakerRegistry,
+) -> Iterator[ModelRouter]:
+    """Create a model router for testing."""
+    reset_model_router()
+    router = ModelRouter(settings=test_settings, circuit_registry=circuit_registry)
+    yield router
+    reset_model_router()
+
+
+@pytest.fixture
+def llm_provider(test_settings: Settings) -> Iterator[LLMProvider]:
+    """Create an LLM provider for testing."""
+    reset_provider()
+    provider = LLMProvider(settings=test_settings)
+    yield provider
+    reset_provider()
+
+
+@pytest.fixture
+def mock_litellm_response() -> dict[str, Any]:
+    """Create a mock LiteLLM response."""
+    return {
+        "id": "test-response-id",
+        "model": "claude-opus-4",
+        "choices": [
+            {
+                "index": 0,
+                "message": {
+                    "role": "assistant",
+                    "content": "This is a test response.",
+                },
+                "finish_reason": "stop",
+            }
+        ],
+        "usage": {
+            "prompt_tokens": 10,
+            "completion_tokens": 20,
+            "total_tokens": 30,
+        },
+    }
+
+
+@pytest.fixture
+def mock_completion_response() -> MagicMock:
+    """Create a mock completion response object."""
+    response = MagicMock()
+    response.id = "test-response-id"
+    response.model = "claude-opus-4"
+
+    choice = MagicMock()
+    choice.index = 0
+    choice.message = MagicMock()
+    choice.message.content = "This is a test response."
+    choice.finish_reason = "stop"
+    response.choices = [choice]
+
+    response.usage = MagicMock()
+    response.usage.prompt_tokens = 10
+    response.usage.completion_tokens = 20
+    response.usage.total_tokens = 30
+
+    return response
+
+
+@pytest.fixture
+def sample_messages() -> list[dict[str, str]]:
+    """Sample chat messages for testing."""
+    return [
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"role": "user", "content": "Hello, how are you?"},
+    ]
+
+
+@pytest.fixture
+def sample_project_id() -> str:
+    """Sample project ID."""
+    return "proj-12345678-1234-1234-1234-123456789abc"
+
+
+@pytest.fixture
+def sample_agent_id() -> str:
+    """Sample agent ID."""
+    return "agent-87654321-4321-4321-4321-cba987654321"
+
+
+@pytest.fixture
+def sample_session_id() -> str:
+    """Sample session ID."""
+    return "session-11111111-2222-3333-4444-555555555555"
+
+
+# Reset all global state after each test
+@pytest.fixture(autouse=True)
+def reset_globals() -> Iterator[None]:
+    """Reset all global state after each test."""
+    yield
+    reset_cost_tracker()
+    reset_circuit_registry()
+    reset_model_router()
+    reset_provider()
+
+
+# Mock environment variables for tests
+# Note: Not autouse=True to avoid affecting default value tests
+@pytest.fixture
+def mock_env_vars() -> Iterator[None]:
+    """Set test environment variables."""
+    env_vars = {
+        "LLM_GATEWAY_HOST": "127.0.0.1",
+        "LLM_GATEWAY_PORT": "8001",
+        "LLM_GATEWAY_DEBUG": "true",
+    }
+    with patch.dict(os.environ, env_vars, clear=False):
+        yield