[Zarr-Support] Route nwbExport through HDF5 backend writer

[ehennestad]

Motivation

This change decouples generated export code from raw HDF5 file identifiers by routing export operations through a backend writer object in order to easier facilitate export to other formats, i.e Zarr.

The generated export methods now accept a writer and call writer methods instead of calling io.writeGroup, io.writeDataset, and io.writeAttribute directly. The handwritten export helpers on the same path were updated to normalize either a writer object or legacy file id input, and the HDF5 path now uses a thin HDF5Writer adapter so behavior stays functionally identical for existing HDF5 export.

This keeps the scope limited to the export path while removing another HDF5-hardcoded seam from the architecture.

How to test the behavior?

addpath(genpath('.'));
results = runtests({'tests.unit.nwbExportTest', 'tests.unit.dataPipeTest', 'tests.unit.linkTest', 'tests.unit.io.WriteTest'});
assertSuccess(results);

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.38%. Comparing base (19239e8) to head (8234daa).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
+io/+backend/BackendFactory.m	80.95%	4 Missing ⚠️
+io/+backend/+base/Writer.m	82.35%	3 Missing ⚠️
+io/+backend/+hdf5/HDF5Writer.m	93.33%	2 Missing ⚠️
+types/+untyped/ObjectView.m	0.00%	2 Missing ⚠️
+types/+untyped/ExternalLink.m	80.00%	1 Missing ⚠️
+types/+untyped/Set.m	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #801      +/-   ##
==========================================
- Coverage   95.44%   95.38%   -0.06%     
==========================================
  Files         203      205       +2     
  Lines        7261     7323      +62     
==========================================
+ Hits         6930     6985      +55     
- Misses        331      338       +7     

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