Registration task not finding reference image in some cases

[jluethi]

We need to improve the _get_matching_ref_acquisition_path_heuristic in the apply registration task.

In a scenario where there are images: [0, 0_illum_corr, 1, 1_illum_corr] and the illum_corr images should be processed, it works for 1_illum_corr. But it fails for 0_illum_corr. The traceback is:

2025-06-03 10:27:35,179; WARNING; No heuristic reference acquisition match found, defaulting to first option 0.
2025-06-03 10:27:35,180; WARNING; Running registration when there are multiple images of the same acquisition in a well. Using a heuristic to match the reference acquisition. Using 0 as the reference image.
Traceback (most recent call last):
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/fractal_tasks_core/tasks/apply_registration_to_image.py", line 400, in <module>
    run_fractal_task(
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/fractal_task_tools/task_wrapper.py", line 66, in run_fractal_task
    metadata_update = task_function(**pars)
                      ^^^^^^^^^^^^^^^^^^^^^
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/pydantic/validate_call_decorator.py", line 60, in wrapper_function
    return validate_call_wrapper(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/pydantic/_internal/_validate_call.py", line 96, in __call__
    res = self.__pydantic_validator__.validate_python(pydantic_core.ArgsKwargs(args, kwargs))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/fractal_tasks_core/tasks/apply_registration_to_image.py", line 121, in apply_registration_to_image
    ROI_table_ref = ad.read_zarr(
                    ^^^^^^^^^^^^^
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/anndata/_io/zarr.py", line 66, in read_zarr
    f = zarr.open(store, mode="r")
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/data/homes/fractal/20230627_joel_fractal/fractal-demos/examples/server/FRACTAL_TASKS_DIR/6/fractal-tasks-core/1.5.1/venv/lib/python3.11/site-packages/zarr/convenience.py", line 139, in open
    raise PathNotFoundError(path)
zarr.errors.PathNotFoundError: nothing found at path ''

The likely culprit:

fractal-tasks-core/fractal_tasks_core/tasks/_zarr_utils.py

Lines 143 to 145 in b8677d4

	# Never return the input `path`
	if path in sorted_path_list:
	sorted_path_list.remove(path)

We don't allow the heuristic to find the image itself and default to 0. Defaulting to 0 worked for many cases, but isn't an overall solution.

I'll investigate if we can just drop this limitation, because I think allowing it to reference itself makes sense here.