test: Add comprehensive unit tests with mocking

Implement complete test coverage for all API endpoints using pytest with proper mocking and dependency injection override patterns.

Test structure:
- tests/test_todo_lists.py: 11 test cases organized by endpoint
- Uses FastAPI TestClient for HTTP testing
- Mock TodoListService to isolate controller logic
- Dependency override pattern for clean testing

Test coverage:
✅ GET /api/todolists (Index)
  - Returns all todo lists successfully
  - Returns empty list when no todos exist

✅ GET /api/todolists/{id} (Show)
  - Returns specific todo list by ID
  - Returns 404 when todo not found

✅ POST /api/todolists (Create)
  - Creates new todo list successfully
  - Validates required fields (422 error)
  - Validates name not empty (422 error)

✅ PUT /api/todolists/{id} (Update)
  - Updates existing todo list successfully
  - Returns 404 when todo not found
  - Validates required fields (422 error)

✅ DELETE /api/todolists/{id} (Delete)
  - Deletes existing todo list (204 response)
  - Returns 404 when todo not found

Testing patterns:
- Fixture-based test client and mock service
- AAA pattern (Arrange, Act, Assert)
- Proper assertion of status codes and response data
- Mock verification with assert_called_once()
- Class-based test organization (TestIndex, TestShow, etc.)

Configuration:
- pytest.ini_options in pyproject.toml
- asyncio_mode = "auto" for async test support
- Strict markers and config enforcement

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

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

Author: cguarinosu

tests/__init__.py

@@ -0,0 +1 @@
+"""Test suite."""

tests/test_todo_lists.py

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+"""Unit tests for TodoList API endpoints."""
+
+from typing import Generator
+from unittest.mock import MagicMock
+
+import pytest
+from fastapi.testclient import TestClient
+
+from app.main import app
+from app.models import TodoList
+from app.services.todo_lists import get_todo_list_service
+
+
+@pytest.fixture
+def mock_service() -> Generator[MagicMock, None, None]:
+    """
+    Create a mock TodoListService for testing.
+
+    Yields:
+        Mock service instance
+    """
+    mock = MagicMock()
+
+    # Override the dependency
+    def override_get_service() -> MagicMock:
+        return mock
+
+    app.dependency_overrides[get_todo_list_service] = override_get_service
+    yield mock
+    app.dependency_overrides.clear()
+
+
+@pytest.fixture
+def client() -> TestClient:
+    """
+    Create a test client for the FastAPI app.
+
+    Returns:
+        TestClient instance
+    """
+    return TestClient(app)
+
+
+class TestIndex:
+    """Tests for GET /api/todolists endpoint."""
+
+    def test_returns_all_todo_lists(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that index returns all todo lists."""
+        # Arrange
+        expected_todos = [
+            TodoList(id=1, name="First list"),
+            TodoList(id=2, name="Second list"),
+        ]
+        mock_service.all.return_value = expected_todos
+
+        # Act
+        response = client.get("/api/todolists")
+
+        # Assert
+        assert response.status_code == 200
+        assert len(response.json()) == 2
+        assert response.json()[0]["id"] == 1
+        assert response.json()[0]["name"] == "First list"
+        assert response.json()[1]["id"] == 2
+        assert response.json()[1]["name"] == "Second list"
+        mock_service.all.assert_called_once()
+
+    def test_returns_empty_list_when_no_todos(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that index returns empty list when no todos exist."""
+        # Arrange
+        mock_service.all.return_value = []
+
+        # Act
+        response = client.get("/api/todolists")
+
+        # Assert
+        assert response.status_code == 200
+        assert response.json() == []
+        mock_service.all.assert_called_once()
+
+
+class TestShow:
+    """Tests for GET /api/todolists/{id} endpoint."""
+
+    def test_returns_todo_list_by_id(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that show returns a specific todo list."""
+        # Arrange
+        expected_todo = TodoList(id=1, name="Test list")
+        mock_service.get.return_value = expected_todo
+
+        # Act
+        response = client.get("/api/todolists/1")
+
+        # Assert
+        assert response.status_code == 200
+        assert response.json()["id"] == 1
+        assert response.json()["name"] == "Test list"
+        mock_service.get.assert_called_once_with(1)
+
+    def test_returns_404_when_not_found(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that show returns 404 when todo list doesn't exist."""
+        # Arrange
+        mock_service.get.return_value = None
+
+        # Act
+        response = client.get("/api/todolists/999")
+
+        # Assert
+        assert response.status_code == 404
+        assert "not found" in response.json()["detail"].lower()
+        mock_service.get.assert_called_once_with(999)
+
+
+class TestCreate:
+    """Tests for POST /api/todolists endpoint."""
+
+    def test_creates_new_todo_list(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that create successfully creates a new todo list."""
+        # Arrange
+        created_todo = TodoList(id=1, name="New list")
+        mock_service.create.return_value = created_todo
+
+        # Act
+        response = client.post("/api/todolists", json={"name": "New list"})
+
+        # Assert
+        assert response.status_code == 201
+        assert response.json()["id"] == 1
+        assert response.json()["name"] == "New list"
+        mock_service.create.assert_called_once()
+
+    def test_validates_required_fields(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that create validates required fields."""
+        # Act
+        response = client.post("/api/todolists", json={})
+
+        # Assert
+        assert response.status_code == 422
+        mock_service.create.assert_not_called()
+
+    def test_validates_name_not_empty(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that create validates name is not empty."""
+        # Act
+        response = client.post("/api/todolists", json={"name": ""})
+
+        # Assert
+        assert response.status_code == 422
+        mock_service.create.assert_not_called()
+
+
+class TestUpdate:
+    """Tests for PUT /api/todolists/{id} endpoint."""
+
+    def test_updates_existing_todo_list(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that update successfully updates an existing todo list."""
+        # Arrange
+        updated_todo = TodoList(id=1, name="Updated list")
+        mock_service.update.return_value = updated_todo
+
+        # Act
+        response = client.put("/api/todolists/1", json={"name": "Updated list"})
+
+        # Assert
+        assert response.status_code == 200
+        assert response.json()["id"] == 1
+        assert response.json()["name"] == "Updated list"
+        mock_service.update.assert_called_once()
+
+    def test_returns_404_when_not_found(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that update returns 404 when todo list doesn't exist."""
+        # Arrange
+        mock_service.update.return_value = None
+
+        # Act
+        response = client.put("/api/todolists/999", json={"name": "Updated"})
+
+        # Assert
+        assert response.status_code == 404
+        assert "not found" in response.json()["detail"].lower()
+        mock_service.update.assert_called_once()
+
+    def test_validates_required_fields(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that update validates required fields."""
+        # Act
+        response = client.put("/api/todolists/1", json={})
+
+        # Assert
+        assert response.status_code == 422
+        mock_service.update.assert_not_called()
+
+
+class TestDelete:
+    """Tests for DELETE /api/todolists/{id} endpoint."""
+
+    def test_deletes_existing_todo_list(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that delete successfully deletes an existing todo list."""
+        # Arrange
+        mock_service.delete.return_value = True
+
+        # Act
+        response = client.delete("/api/todolists/1")
+
+        # Assert
+        assert response.status_code == 204
+        assert response.content == b""
+        mock_service.delete.assert_called_once_with(1)
+
+    def test_returns_404_when_not_found(
+        self, client: TestClient, mock_service: MagicMock
+    ) -> None:
+        """Test that delete returns 404 when todo list doesn't exist."""
+        # Arrange
+        mock_service.delete.return_value = False
+
+        # Act
+        response = client.delete("/api/todolists/999")
+
+        # Assert
+        assert response.status_code == 404
+        assert "not found" in response.json()["detail"].lower()
+        mock_service.delete.assert_called_once_with(999)