feat: Auto-cleanup test containers on failure with --keep-containers flag

[gerchowl]

Problem

Test containers are left behind when tests fail because fixture cleanup doesn't always run on hard failures or interrupts. This causes:

• Resource accumulation
• make test failures due to lingering container detection
• Manual cleanup required after debugging

Proposed Solution

1. Add pytest option --keep-containers

def pytest_addoption(parser):
    parser.addoption(
        "--keep-containers",
        action="store_true",
        default=False,
        help="Keep test containers after test completion (for debugging failed tests)",
    )

2. Use request.addfinalizer() for reliable cleanup

Replace yield-based cleanup with addfinalizer() which runs even on test failures:

@pytest.fixture(scope="function")
def devcontainer_up(initialized_workspace, request):
    # ... setup code ...
    
    def cleanup():
        if request.config.getoption("--keep-containers"):
            print(f"\n⚠️  Keeping container for debugging: {container_name}")
            return
        # ... existing cleanup code (podman compose down, etc.) ...
    
    request.addfinalizer(cleanup)
    
    yield workspace_path

Usage

# Normal run - auto-cleanup on success AND failure
make test-integration

# Debug mode - keep containers for investigation  
uv run pytest tests/test_integration.py --keep-containers -x

# Inspect the failed container
podman exec -it <container_name> bash

# Clean up manually when done
make clean-test-containers

Benefits

• Containers cleaned up reliably even on failures
• --keep-containers flag for debugging
• Works with -x (stop on first failure) for debugging
• No more manual cleanup after failed test runs

Affected Fixtures

• devcontainer_up
• devcontainer_with_sidecar
• initialized_workspace
• Any other fixtures that create containers