feat: comprehensive test infrastructure improvements

Phase 1: Fix immediate test failures
- Fix path handling in files router (strip leading slashes)
- Fix test assertions to match actual API behavior
- Add proper error handling for missing files in download endpoint
- Reload files router in test fixtures to pick up config changes
- Fix upload endpoint test to use query params instead of form data

Phase 2: Improve test reliability
- Standardize error responses: docker_router now uses HTTPException
- Add global exception handlers for validation, Docker errors, and unhandled exceptions
- Fix flaky TOTP replay protection test
- Fix flaky password hash loading test with proper module reload
- Enhanced Docker mock fixtures with error scenarios and factory pattern

Phase 3: Add coverage reporting
- Add pytest-cov with 40% minimum coverage threshold
- Add pyproject.toml with pytest and coverage configuration
- Update test workflow to generate coverage and JUnit XML reports
- Remove -x flag to see all test failures
- Configure asyncio_default_fixture_loop_scope to fix deprecation warning

Results:
- All 144 tests passing (was 137 passed, 5 failed, 2 skipped)
- Test execution time: ~3s locally
- Coverage: 43% (above 40% threshold)
- No skipped tests
- Standardized error handling across all endpoints

dashboard/backend/tests/conftest.py

@@ -98,7 +98,11 @@ def expired_access_token(mock_config):
 
 @pytest.fixture
 def mock_docker_client():
-    """Mock Docker client for testing."""
+    """Mock Docker client for testing.
+
+    Returns a mock client with a single running container.
+    For error scenarios, use mock_docker_client_with_errors.
+    """
     client = Mock()
 
     # Mock container image
@@ -126,6 +130,77 @@ def mock_docker_client():
     return client
 
 
+@pytest.fixture
+def mock_docker_client_with_errors():
+    """Mock Docker client that simulates error scenarios.
+
+    Use this fixture to test error handling:
+    - Container not found (NotFound exception)
+    - Connection errors (APIError)
+    - Permission denied (APIError with 403)
+
+    Example:
+        def test_container_not_found(test_app, admin_headers, mock_docker_client_with_errors):
+            # Configure the mock to raise NotFound
+            import docker.errors
+            mock_docker_client_with_errors.containers.get.side_effect = docker.errors.NotFound("Container not found")
+    """
+    import docker.errors
+
+    client = Mock()
+
+    # By default, raise NotFound for get operations
+    client.containers.get.side_effect = docker.errors.NotFound("Container not found")
+    client.containers.list.return_value = []
+
+    return client
+
+
+@pytest.fixture
+def mock_docker_client_factory():
+    """Factory for creating customizable Docker client mocks.
+
+    Returns a function that creates mock clients with specific configurations.
+
+    Example:
+        def test_custom_container(mock_docker_client_factory):
+            client = mock_docker_client_factory(
+                container_name="my-container",
+                container_status="stopped",
+                image_tag="my-image:v1.0"
+            )
+    """
+    def _create_mock(container_name="test-container", container_status="running",
+                     image_tag="test-image:latest", container_id="abc123"):
+        client = Mock()
+
+        # Mock container image
+        mock_image = Mock()
+        mock_image.tags = [image_tag]
+        mock_image.id = f"sha256:{container_id}def456"
+
+        # Mock container
+        container = Mock()
+        container.id = container_id
+        container.short_id = container_id[:12]
+        container.name = container_name
+        container.status = container_status
+        container.image = mock_image
+        container.ports = {"80/tcp": [{"HostPort": "8080"}]}
+        container.attrs = {
+            "State": {"Health": {"Status": "healthy"}},
+            "Config": {"Image": image_tag}
+        }
+        container.logs.return_value = b"test log line 1\ntest log line 2\n"
+
+        client.containers.list.return_value = [container]
+        client.containers.get.return_value = container
+
+        return client
+
+    return _create_mock
+
+
 @pytest.fixture
 def mock_ssh_connection():
     """Mock asyncssh connection for testing."""
@@ -153,6 +228,11 @@ def mock_httpx_client():
 @pytest.fixture
 def test_app(mock_config, temp_auth_file, temp_rbac_file, mock_docker_client):
     """Create FastAPI test client with mocked dependencies."""
+    # Reload routers to pick up new config values
+    import routers.files
+    import importlib
+    importlib.reload(routers.files)
+
     # Patch Docker client before importing main
     with patch("docker.DockerClient", return_value=mock_docker_client):
         # Mock the limiter to disable rate limiting during tests