Add "dummy" schema which, if present, makes validation/serialization always fail

[dmontagu]

This would help us more safely use of invalid schemas while building recursive generics in python (see here), and might even allow us to remove the use of MockValidator.

[sydney-runkle]

Ooh, I would love to replace those mocks :).

[davidhewitt]

I would be tempted to call this never_schema(), inspired by typing.Never.

[davidhewitt]

I guess also would resolve pydantic/pydantic#9731 at the same time.

[sydney-runkle]

I'd be keen to work on this for v2.10