Fix HERD.get_object_entities(attribute=...) to mirror add_ref(attribute=...)

[rly]

Motivation

Fixes #1501.

HERD.add_ref(container=..., attribute=...) and HERD.get_object_entities(container=..., attribute=...) resolved the attribute argument differently, so a reference added with an attribute could not be read back with the same attribute:

• add_ref routes through _validate_object, which for a non-DataType attribute (e.g. DynamicTable.description) stores the parent container under a computed relative_path.
• get_object_entities instead did container[attribute] with the default relative_path='', which raises KeyError (or TypeError for containers without __getitem__) for non-DataType attributes.

Solution

_validate_object now takes a create flag and threads it to its _check_object_field calls. get_object_entities reuses _validate_object(..., create=False) for the attribute case, so both methods resolve the attribute identically.

The attribute=None path in get_object_entities is unchanged, so directly passing relative_path still works.

How to test the behavior

Added test_get_obj_entities_non_datatype_attribute: adds a ref with attribute='description' and retrieves it with the same attribute. It fails on dev (KeyError: 'description') and passes with this change. The existing test_get_obj_entities_attribute (DataType-attribute / column case) continues to pass.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This will be checked during CI.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number?

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (54dbf42) to head (2f2a319).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1504   +/-   ##
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  Coverage   93.24%   93.24%           
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  Files          41       41           
  Lines       10217    10217           
  Branches     2107     2107           
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  Hits         9527     9527           
  Misses        415      415           
  Partials      275      275           

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[oruebel]

I added some suggestions to help clarify the usage of _check_object_field and fix what I believe is a bug in get_key