Abort when ggproto definition is circular

[teunbrand]

This PR aims to fix #6583.

AFAICT, we cannot detect this during object construction.
The second best way to detect this is in fetch_ggproto(), as it is such a widely used accessor.
Reprex from issue:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

A <- ggproto("A")
A <- ggproto(NULL, A)
A
#> Error in `fetch_ggproto()` at ggplot2/R/ggproto.R:172:3:
#> ! <A> cannot have a circular definition.

^{Created on 2025-09-25 with reprex v2.1.1}