[QTI] Convert legacy assessment items to QTI on the fly to avoid frontend backward compatibility handling

[rtibbles]

Overview

Build the reusable legacy→QTI conversion (structured single/multiple selection, input, true/false → <qti-assessment-item> XML) and serve it through the API layer (dual-read) so the frontend only ever receives QTI. The one-time global backfill is #11.

Complexity: High
Target branch: unstable

Context

• Legacy items store structured question/answers with legacy types; the existing pydantic models already generate QTI for choice + text-entry — the legacy-expressible set.
• A uniform QTI model removes publish-time format branching and the editor's legacy-edit path ([QTI] Support legacy assessment item types in the QTI Editor #5980 becomes unnecessary).
• The transitional conversion lives in the API layer (dual-read), not the database — no schema change.

The Change

• Provide a reusable legacy→QTI conversion producing <qti-assessment-item> XML from structured items.
• During a short transition window, the API endpoint converts legacy items to QTI on read, so the frontend only ever receives QTI.
• Leave perseus_question items as raw Perseus — out of scope here; handled by mixed publishing (Browserify #8).

Acceptance Criteria

• A reusable conversion produces <qti-assessment-item> XML from each supported legacy type (single/multiple selection, input, true_false)
• The conversion is lossless: prompt, choices/answers, correctness, hints, and media survive
• Each converted item validates against the QTI 3.0 XSD (Ant's backend plus setting up front end files #2)
• During the transition window the API endpoint returns QTI for a still-legacy row (dual-read)
• Items that fail conversion are surfaced, not silently dropped
• Tests cover conversion per legacy type and the API dual-read

References

• [QTI] Support legacy assessment item types in the QTI Editor #5980 — legacy types in the QTI editor (superseded once this lands)

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.