[MLv2] Test `metabase.lib.convert` by round-tripping every `mbql-query` (#29487)

This is a great way to make sure the conversion in both directions
is robust; every feature of MBQL is exercised somewhere in the BE tests.

In a few cases the `mbql-query` is called from a context where a
`(is ...)` test won't work - those places can use a `mbql-query-no-test`
instead. There were actually only two of these.

This exposed several issues in `->legacy-MBQL` which have now been fixed.