Make HERDManager an abstract interface

[rly]

Motivation

HERDManager was previously a concrete mixin that used a private _herd attribute with a manual property, bypassing the __fields__ / child-tracking system entirely. This PR refactors it into an abstract interface (ABC) that:

1. Defines an abstract external_resources property, requiring subclasses to provide the implementation
2. Subclasses satisfy the interface by declaring external_resources in their __fields__ (or __nwbfields__), which triggers auto-generated getter/setter via __gather_fields
3. Serves as a marker for isinstance checks in HERD and HDMFIO

This design is cleaner because:

• The mixin doesn't dictate field configuration — subclasses can set required_name, child, etc. as needed
• external_resources can now be a proper child field with parent tracking in container subclasses like NWBFile

__gather_fields is updated to allow auto-generated properties to override inherited abstract properties, clearing __abstractmethods__ accordingly.

Additionally, link_resources and get_external_resources methods are added to HERDManager so that any subclass can manage a secondary linked HERD alongside the primary one. This moves functionality that was previously NWBFile-specific into the base class for broader reuse.

This PR will make the corresponding PyNWB PR NeurodataWithoutBorders/pynwb#2111 cleaner.

How to test the behavior?

from hdmf.container import Container, HERDManager
from hdmf.common.resources import HERD

class MyFile(HERDManager, Container):
    __fields__ = (
        {'name': 'external_resources', 'child': True, 'required_name': 'external_resources'},
    )

f = MyFile(name='test')
f.external_resources = HERD()
assert f.external_resources.parent is f
assert isinstance(f, HERDManager)

# Linked HERD support
linked = HERD()
f.link_resources(linked)
assert f.get_external_resources() is f.external_resources  # primary
assert f.get_external_resources(linked=True) is linked      # linked

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?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.13%. Comparing base (c7dcb5f) to head (e115bce).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1431      +/-   ##
==========================================
+ Coverage   93.10%   93.13%   +0.02%     
==========================================
  Files          41       41              
  Lines       10038    10047       +9     
  Branches     2061     2063       +2     
==========================================
+ Hits         9346     9357      +11     
+ Misses        416      415       -1     
+ Partials      276      275       -1     

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[rly]

Optional tests, pynwb tests, and hdmf-zarr tests are expected to fail until the PyNWB 4 release.