Add HERD repr/_repr_html_ surfacing references as a flattened table

[rly]

Motivation

Fixes #1508.

HERD inherits the generic Container._repr_html_, which renders its six child tables as nested collapsibles. For a HERD read back from a file these can appear empty at first glance, even though the data is present and accessible via to_dataframe() and the individual tables. The PyNWB HERD tutorial (NeurodataWithoutBorders/pynwb#2200) currently has to warn users about this. The underlying need was raised in NeurodataWithoutBorders/nwb-schema#652.

Solution

Add a HERD-specific __repr__ and _repr_html_ that show a one-line summary (table sizes) plus the flattened to_dataframe() view (one row per object/key/entity association). The display falls back gracefully:

• empty HERD: shows "No external resource references." (to_dataframe raises when there are no object-key rows, so the helper guards on that);
• closed backing file or any to_dataframe failure: the existing closed-file warning banner is preserved and the table is omitted rather than raising (a repr must never raise).

Display only; no change to stored data or table APIs.

Screenshot

The _repr_html_ of a populated HERD, rendered as a Jupyter output cell:

How to test the behavior

Added test_repr_empty and test_repr_populated in tests/unit/common/test_resources.py (empty HERD reprs do not raise and say "No external resource references"; a populated HERD surfaces the summary line and the entity content in the HTML repr). Full tests/unit/common/test_resources.py passes (63 passed, 10 skipped).

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

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.19%. Comparing base (abd0a7f) to head (77e4530).
⚠️ Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/hdmf/common/resources.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1510   +/-   ##
=======================================
  Coverage   93.19%   93.19%           
=======================================
  Files          41       41           
  Lines       10191    10221   +30     
  Branches     2107     2110    +3     
=======================================
+ Hits         9497     9525   +28     
- Misses        416      418    +2     
  Partials      278      278           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.