When the move of an attachment fails, a redirection to an attachment that was never created is left behind

When an attachment is moved to another page with Create a redirection checked and the save of the target page fails, the move is rolled back — the attachment is put back on the source page — but the redirection object that was created on the source page is not removed. The source page is therefore left with a redirection pointing to an attachment that was never created.

Since a redirection is resolved before the attachment is served, and the resolution only checks that a redirection object exists (not that the attachment it points to exists), the attachment restored by the rollback also becomes unreachable: downloading it from the source page redirects to the target attachment, which does not exist.

Steps to reproduce:

1. Create a page A holding an attachment file.txt, and a page B.
2. Arrange for the save of B to fail (a storage error, for instance) — this is the hard part, the problem is not reachable through the UI without a failing save.
3. From A, move file.txt to B, keeping Create a redirection checked.

Expected result:

• Once the move has been rolled back, A holds file.txt and no redirection object.

Actual result:

• A holds file.txt again (the rollback itself works), but it also holds a RedirectAttachmentClass object pointing to file.txt in B, which was never created.
• Downloading file.txt from A redirects to the non-existent attachment in B instead of serving the restored attachment.

Notes:

• Only the move-to-another-page path is affected. Renaming an attachment inside the same page performs a single save and has no rollback.
• MoveAttachmentJob#initializeAutoRedirection adds the object to the source document before the first save, and the rollback branch of MoveAttachmentJob#transactionalMove restores the attachment but does not remove that object.
• Present since the move-attachment feature was introduced in 14.0 (XWIKI-1657).
• The current behaviour is pinned by MoveAttachmentJobTest#processTargetSaveFail, added by XWIKI-24680, so fixing this requires updating that test.