As far as I can tell there's no way to pass nullability annotations through a generic in Java. To allow for expressing nullability of return values, we should make a breaking change to add a new NullableResult<T>, which is the same as Result<T> except for the annotations we put on the argument to success. (The breaking part being that some methods that currently have a Result parameter will change to a NullableResult parameter.)

For now I'm suppressing the UnknownNullability warning on Result<T>'s success method.

@tarrinneal I think we should do this at the same time we replace Reply with Result for #118243 since we'll be making a related breaking change for Java at that point (although this one will affect more code).