Nested spaces migrator does not protect against XWIKI-13402

The nested spaces migrator reorganizes the documents based on their parent-child relation, without handling in any way the length of the fullnames of documents generated by this reorganization.

It's enough for one page to have a long name and its whole subtree will not be migratable.

A preview could be provided for the pages that would error because of XWIKI-13402 , and the executor of the migration could get the opportunity to rename the document in question or reorganize the hierarchy.