[QTI] Serve legacy assessment items as QTI through the API endpoint (dual-read)

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Overview

During a short transition window, the AssessmentItem API endpoint converts legacy items to QTI on read — via the reusable conversion (#6003) — so the frontend only ever receives QTI and needs no backward-compatibility handling. The one-time global backfill that makes this permanent and removes the dual-read is #6007.

Complexity: Medium
Target branch: unstable

Context

• The reusable legacy→QTI conversion is #6003; this issue wires it into the endpoint's read path.
• The transitional conversion lives in the API layer (dual-read), not the database — no schema change.
• Leave perseus_question items as raw Perseus — out of scope here; handled by mixed publishing (#6006).

The Change

• On read, the AssessmentItem endpoint returns QTI (type='QTI', raw_data XML) for a still-legacy row by converting it via #6003.
• Pass perseus_question rows through unchanged.
• The dual-read is removed once the global backfill (#6007) lands.

Acceptance Criteria

• The API endpoint returns QTI for a still-legacy row (dual-read), so the frontend only ever receives QTI
• perseus_question rows are returned unchanged
• Conversion failures are surfaced, not silently dropped
• Tests cover the API dual-read across the supported legacy types

References

• #6003 — reusable legacy→QTI conversion (consumed here)
• #6007 — global backfill that later removes this dual-read
• #5980 — legacy types in the QTI editor (superseded once this lands)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.