Looping over models during save_results doesn't work for non-list Sequence

[emolter]

A non-list but list-like object (e.g., the JWST ModelContainer) should be saved in the same way as a list of models if it does not have its own save() method. However, this is not what currently happens, because

stpipe/src/stpipe/step.py

Line 544 in 02840fe

if not isinstance(step_result, list | tuple):

checks specifically for list, not anything that looks like a list.

See the unit tests below.

def test_save_list(tmp_cwd, model_list):
    """ensure all models are saved using the model's save method. This test passes"""
    step = StepWithModel()
    step.run(model_list)
    for i in range(3):
        assert (tmp_cwd / f"test{i}-saved.txt").exists()

def test_save_container(tmp_cwd, model_list):
    """ensure list-like save still works for non-list sequence.

    This test fails on an AssertionError. step runs fine, but nothing is saved at all.
    Behavior should be changed so that this test passes.
    """
    container = SimpleContainer(model_list)
    step = StepWithModel()
    step.run(container)
    for i in range(3):
        assert (tmp_cwd / f"test{i}-saved.txt").exists()

Note these make use of the SimpleContainer and StepWithModel classes here:

stpipe/tests/test_step.py

Line 493 in 53ca364

class SimpleContainer(Sequence):