Fix jsonrpc response None result missing

[kingosticks]

I appear to have left all the tests broken

[kingosticks]

Before I fix the tests, do you want this or do you want to take it in another direction?

[jodal]

Fixing the tests is probably just about replacing Response with the right one of the two alternatives. However, if we do .decode(..., type=Response) anywhere in the implementation, that will not work without adding some tag field to the types (which is not in our power to change) to be able to differentiate them.

I think an alternative approach is to change the Response class to use another default value, so that the right fields are omitted in the serialization of each case:

class Response(msgspec.Struct, kw_only=True, omit_defaults=True):
    jsonrpc: Literal["2.0"]
    id: RequestId | None  # None is allowed, but it must be set explicitly.
    result: Any | None | msgspec.UNSET = msgspec.UNSET
    error: ErrorResponseDetails | msgspec.UNSET = msgspec.UNSET

    @classmethod
    def as_success(cls, id: RequestId, result: Any | None) -> Self:
        return cls(jsonrpc="2.0", id=id, result=result)

    @classmethod
    def as_error(cls, id: RequestId | None, error: ErrorResponseDetails) -> Self:
        return cls(jsonrpc="2.0", id=id, error=error)

[kingosticks]

I was struggling with what to use as an alternative default, msgspec.UNSET is much neater. I prefer your fix.

[jodal]

Implemented in 0fde81e.