Restoring a published QTI item stores lxml's reserialization instead of the published XML #6305

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Target branch: unstable

Observed behavior

• Restore stores a QTI item's raw_data by passing the packaged XML through rewrite_qti_media_paths (import_tools.py#L585-L588, from Restore QTI exercises when restoring a channel #6243).
• Since Parse QTI items to rewrite them at publish instead of matching regexes #6241, rewrite_qti_media_paths reparses the item and returns lxml's serialization (media.py#L33-L34), so the restored raw_data differs from the source item's.
• The two PRs merged minutes apart; the Python unit tests job has failed on unstable since then, and on every PR based on it.

Errors and logs

FAILED test_restore_channel.py::QTIRestoreTestCase::test_dual_published_node_restores_each_question_once
FAILED test_restore_channel.py::QTIRestoreTestCase::test_qti_only_node_restores_editor_and_legacy_items_as_qti
SUBFAILED(order=1) test_restore_channel.py::QTIRestoreTestCase::test_perseus_question_in_package_restores_as_perseus_question
E   AssertionError: '<?xml version=\'1.0\' encoding=\'UTF-8\'?>\n<qti-assess[1250 chars]tem>' != '<?xml version="1.0" encoding="UTF-8"?><qti-assessment-i[1247 chars]tem>'
contentcuration/contentcuration/tests/test_restore_channel.py:417: AssertionError

User-facing consequences

• CI is red for every Studio PR, so real regressions in the Python suite are hidden.

Steps to reproduce

1. Check out unstable at 51657a6d9.
2. Run pytest contentcuration/contentcuration/tests/test_restore_channel.py -k QTIRestore.

Acceptance Criteria

• pytest contentcuration/contentcuration/tests/test_restore_channel.py passes on unstable.
• A restored QTI or Perseus-question item's raw_data equals its source item's raw_data.
• Republishing a restored node produces package item files equal to the source package's.

AI usage

Claude Code found the failure while fixing CI on #6273, bisected it to the #6241/#6243 merge, and drafted this issue.