[QTI] QTI 3.0 schema XML validation utilities

[rtibbles]

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Overview

One authoritative backend validator that checks any QTI item against the QTI 3.0 schema, independent of source. The frontend surfaces only user-correctable errors; full compliance is guaranteed here.

Complexity: Medium
Target branch: unstable

Context

• QTI arrives from the editor and from ricecooker, and may use interaction types or constructs the editor never produces.
• The sync API accepts writes outside the editor, so the frontend's by-construction validity can't be assumed; QTI must be validated on save, not only at upload and publish.
• Publish validates every item in a channel and ricecooker uploads in bulk, so the schema can't be re-parsed per item.
• The pydantic models ([QTI] Build the QTI declaration model with XML parsing and serialization #5965) cover a restricted generation subset, not the full spec.

The Change

• Provide a reusable backend entry point that validates a QTI item against the QTI 3.0 schema and returns pass/fail plus actionable errors.
• Compile the schema once and reuse it across items, so bulk publish and upload stay efficient.
• Keep it a standalone validator; the entry points wire it in — save (Introducing Content Curation #1), upload (Edit, preview, trash page created #5), publish (adds url to test page for rendering handlebars templates #3).

Acceptance Criteria

• A reusable function validates a QTI item against the QTI 3.0 schema, returning structured pass/fail + errors
• Valid QTI passes, including interaction types the editor does not author
• Malformed or non-compliant QTI fails with errors that identify the problem
• The compiled schema is reused across items, not re-parsed per item
• Tests cover valid items across multiple interaction types and representative invalid items

References

• QTI 3.0 specification: https://www.imsglobal.org/spec/qti/v3p0/impl
• [QTI] Build the QTI declaration model with XML parsing and serialization #5965 — pydantic QTI model

AI usage

Architecture decided with the maintainer across an iterative session: blanket QTI type with the item XML in raw_data; XSD-authoritative validation across all sources; a legacy→QTI global migration with an API-layer dual-read; ricecooker upload delegating to the AssessmentItem serializer; and a Perseus custom-interaction contract confirmed against the QTI 3.0 specification. Claude mapped the existing publish/validation/ricecooker code, proposed the breakdown, and drafted each issue; the maintainer steered every decision and reviewed throughout.