Support XML Attributes as Searchable Metadata in the PDS Registry

[jordanpadams]

Summary

PDS4 labels encode metadata in both element text content and XML attributes. The PDS Registry currently indexes only element text content — all XML attributes (e.g., xsi:nil, nilReason, unit, and any discipline/mission dictionary attributes) are silently dropped during harvest and are therefore neither stored in the registry nor searchable via the API.

This theme covers the end-to-end work to support XML attributes as first-class metadata in the PDS Registry ecosystem, from harvest-time extraction through API-level search and retrieval.

In Scope

• Harvest extraction and indexing of XML attributes from PDS4 label elements
• Registry API search, filter, and response representation of XML attribute fields
• Handling of xsi:nil="true" as a specific high-priority case (per PDS4 CCB discussion)

Related Issues

• https://github.com/NASA-PDS/PDS4-CCB/issues/95 — CCB discussion on nil attributes and empty classes
• NASA-PDS/harvest#333 — Harvest: index XML attributes from PDS4 label elements
• As a data user, I want to search and filter on XML attribute values from PDS4 labels #789 — Registry API: search and filter on XML attribute values

🦄 Related requirements

NASA-PDS/harvest#333
#789