[QTI] Editing a question keeps the interaction attributes the editor doesn't set #6287

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Target branch: unstable

Observed behavior

The first edit to a QTI question rewrites its interaction element with only the attributes its builder sets. Any other attribute is dropped.

• Text entry: buildTextEntryInteractionXML writes only response-identifier and expected-length.
• Every legacy input question converted by convert.py carries placeholder-text="Enter your answer here", which is lost.
• Choice: the builder drops class, so class="qti-labels-upper-alpha" is lost.

Errors and logs

None.

Expected behavior

An edit keeps every attribute on the interaction element that the editor doesn't set.

User-facing consequences

Learners see a question that differs from the one uploaded, e.g. an answer field without its placeholder, or choices without their labels.

Steps to reproduce

1. Open an exercise with a legacy input question that was converted to QTI.
2. Click Edit and change a hint.
3. Inspect the emitted raw_data: <qti-text-entry-interaction> has no placeholder-text.

Acceptance Criteria

• Editing a converted text entry question keeps placeholder-text.
• Editing a choice question keeps a class on <qti-choice-interaction>.
• Tests cover both cases through parse, build and assemble.

AI usage

Claude Code found this while reviewing the #6225 branch and drafted this issue. Both losses were reproduced under Jest by parsing an item, rebuilding it with its descriptor and assembling it.