fix(datatype): convert from sqlglot VARCHAR(MAX) correctly

[NickCrews]

Fixes #11191

I'd like to add a test for this. I think checking for roundtripping ibis.string -> sqglot.DataType -> ibis.string across all backends sounds like the best approach.

[NickCrews]

Ah, it looks like there is already test_roundtripable_types() in ibis/backends/sql/tests/test_datatypes.py, but it only uses the superclass SqlGlotType type mapper, not the backend-specific type mappers. Adjusting this now.

[NickCrews]

eh, the roundtrip test doesn't actually catch the bug, since ibis.String goes to VARCHAR(max), but the bug only appears when using NVARCHAR(max). So at this point I didn't actually add a test, it didn't seem worth it. Maybe could add test only to the mssql backend tests?

Another implementation we could do here is to hoist the MSSQL implementation, with the safety check for "MAX" length, up to the superclass SqlglotType. Then there will less code, but the needs of the different backends will get mixed with each other. This implementation is more explicit and granular and extensible.

[cpcloud]

Thanks!