[codex] Route generated export through backend writer

[ehennestad]

Motivation

This change decouples generated export code from raw HDF5 file identifiers by routing export operations through a backend writer object.

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?