Fix regression in colour/fill scale name redirection

[teunbrand]

This PR aims to fix #6623.

Briefly, it populates named parameters to the scale constructors such that name is once again populated by the first unnamed argument (or the argument named 'name', of course).

Reprex from issue, note that all legends have the given names, rather than default labels:

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

ggplot(mtcars, aes(x = disp, y = hp, size = gear, colour = wt, fill = factor(cyl))) +
  geom_point(shape = 21) +
  scale_fill_discrete("Cylinders") + 
  scale_colour_continuous("Weight") + 
  scale_size_continuous("Gears")  

^{Created on 2025-09-24 with reprex v2.1.1}

[thomasp85]

LGTM