Do not fill an absent named link with links of a matching type

[bendichter]

Motivation

Fix #1595.

On read, ObjectMapper.__get_subspec_values assigns a named link from the group's link of that name. If the file has no link with that name, the else branch also ran for named specs and matched the group's other links by target_type. Since #1481, subtypes match too, so an optional named link whose target type is a common base type got every other link in the group as a list. Construction then failed.

DANDI dandiset 001878 hits this. Its EthogramBouts (ndx-ethogram) declares optional links source (target NWBDataInterface), source_pose, source_video, observation_intervals and ethogram. The file sets all of them except source:

ConstructError: ... "Could not construct EthogramBouts object due to: CustomClassGenerator.set_init.<locals>.__init__: incorrect type for 'source' (got 'list', expected 'NWBDataInterface')"

This PR matches links by type only for unnamed link specs (else: → elif subspec.name is None:). A named link that's absent from the file is left unset.

How to test the behavior?

pytest tests/unit/build_tests/test_io_map.py -k MissingNamedLink

The new test fails on dev (source is set to the other link's target) and passes with this change. The full unit suite shows no new failures. The DANDI file (https://dandiarchive.s3.amazonaws.com/blobs/eaa/b5c/eaab5c37-7a7f-4b65-864d-f2986df5d64e) reads with pynwb 4.2.0 on this branch.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (a7e0ec3) to head (080352c).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1598      +/-   ##
==========================================
- Coverage   93.17%   93.16%   -0.01%     
==========================================
  Files          41       41              
  Lines       10304    10305       +1     
  Branches     2132     2133       +1     
==========================================
  Hits         9601     9601              
  Misses        423      423              
- Partials      280      281       +1     

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