Handle snapshot case in deletion vector conversion

It is also important to cover the snapshot case since that is how the first sync happens for a table. If the first sync does not carry through the deletion vectors, that could lead to some correctness issues.

_Originally posted by @the-other-tim-brown in https://github.com/apache/incubator-xtable/pull/634#discussion_r1938084202_

No file or test is named. Start by locating the deletion vector conversion entry point and the snapshot path used for a table's first sync; trace whether deletion vectors are preserved there. Done means the snapshot conversion carries deletion vectors correctly, with coverage for that case.

Written by the indexing model from the issue text.