fix: handle uninitialised runtime for get_artifact_warehouse

[bgrey001]

@ciaran-coleman raised an issue when loading acquisitions in a notebook.

mosaic.get_artifact_warehouse() raised AttributeError: module mosaic has no attribute get_artifact_warehouse when called before mosaic.run() / mrun had initialised the runtime (e.g. from a notebook calling problem.acquisitions.load('acquisitions.h5').

Cause: the call was routed through mosaic.__getattr__ → getattr(None, ...).

Fix: add a top-level function that returns None when no runtime exists, otherwise delegates to getattr.

Tested using the following (before fix the bug is reproduced, post fix the test passes):

@pytest.fixture
def no_runtime():
    """Force mosaic._runtime to None and restore on teardown."""
    import mosaic
    saved = mosaic._runtime
    mosaic._runtime = None
    try:
        yield mosaic
    finally:
        mosaic._runtime = saved


def test_get_artifact_warehouse_returns_none_when_runtime_is_none(no_runtime):
    """The notebook case: ``mosaic.get_artifact_warehouse()`` must not raise."""
    assert no_runtime.get_artifact_warehouse() is None