QTI editor drops prefixed MathML from a question prompt on save #6308

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Observed behavior

• A QTI item whose prompt carries MathML under a prefix (xmlns:m on the root, <m:math> in the prompt) loses the formula on its first save from the editor. Only its text survives.
• Source prompt: <qti-prompt>solve <m:math><m:mi>x</m:mi><m:mo>+</m:mo><m:mn>1</m:mn></m:math>.</qti-prompt>
• Saved prompt after typing at the end: <qti-prompt><p>solve x+1. Edited</p></qti-prompt>
• adoptNode re-creates foreign elements with createElementNS(namespace, node.tagName), so the HTML handed to TipTap contains <m:math>, which TipTap does not recognise as math.

Expected behavior

• The formula in the prompt survives the edit.

User-facing consequences

• Imported items that use prefixed MathML silently lose their formulas the first time anyone edits the question text, and publish without them.

Steps to reproduce

1. Seed an exercise with a QTI item whose root declares xmlns:m="http://www.w3.org/1998/Math/MathML" and whose single-choice prompt contains <m:math>…</m:math>.
2. Open the exercise's Questions tab, click "Edit" on the item, type at the end of the question text, click "Close".
3. Read the saved raw_data: the <m:math> element is gone.

Seen on unstable at 90c5b61, during manual QA for #6290.

Acceptance Criteria

• Editing the prompt of an item with <m:math> (prefixed MathML) saves a prompt that still contains the formula as MathML.
• The saved item passes the QTI XSD check.
• A test covers it, from opening the item to the emitted raw_data.

AI usage

Claude Code found this during manual QA of #6290 and drafted the issue.