Add serializer for AssertRefSnapshotId allowing null json value

[ox]

Closes #2342

Rationale for this change

The last attempt at fixing this (#2343) did not actually end up serializing None values to null. This was b/c the AssertRefSnapshotId requirement was often a child object of some other Request object, which was actually getting the model_dump_json() call. This meant that the exclude_none=True override would remove any Nones before any of the Field-specific configurations would be considered (like exclude=False or the model's own model_dump_json method).

I then investigated setting some ConfigDict settings on the model, but that also did not have any useful configurations.

I looked into setting exclude_none=True at all of the model_json_dump() callsites but there were too many and it would be easy to forget to set that value every time. The issue is that many values are supposed to be absent to instruct the REST Catalog about what the client wants (things like filtering, for instance).

The solution I ended up with was allowing snapshot_id to be a required int with a default value of -1, and adding a json-specific field serializer to write out null when dumping to json. This seems to work

The solution that worked and allowed for snapshot_id to be set to None (as it's done in Ray Data), was to define a model_serializer method and always return the fields. This was based on conversations in pydantic/pydantic#7326.

Are these changes tested?

Yes

Are there any user-facing changes?

Actually allow snapshot-id to be null for assert-ref-snapshot-id requirements

[kevinjqliu]

LGTM! slight nit on making a few changes for maintainability :)

[kevinjqliu]

nit: is there a way to call super() or the default serializer? we just want to explicitly override for snapshot-id.

otherwise, we'd have to remember to change to this function everytime we add a new variable

[ox]

yes! Added the mode='wrap' param that allows that

[kevinjqliu]

LGTM! very clean, very nice

[Fokko]

Thanks for following up here @ox 🙌

[kevinjqliu]

Thanks for the follow up PR @ox and thanks @Fokko for the review