Allow server decorators to postprocess `ValidationException` not attached error messages (#2338)

Should they want to, a server decorator can now postprocess the error
message that arises when a constrained operation does not have the
`ValidationException` shape attached to its errors.

This commit adds a test to ensure that when such a decorator is
registered, the `ValidationResult` can indeed be altered, but no such
decorator is added to the `rust-server-codegen` plugin.