Fix guide_axis_theta() expression label bug

[teunbrand]

I apologise for not filing an issue first, but I ran into the following bug.
On the main branch, expressions aren't rendered properly with guide_axis_theta(). You can see in the example below, the subscript and superscript aren't drawn appropriately.

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

ggplot(data.frame(x = 1:2), aes(x, x)) +
  geom_point() +
  coord_radial() +
  scale_x_continuous(
    breaks = c(1.2, 1.8),
    labels = expression(A^2, B[2])
  )

^{Created on 2023-12-12 with reprex v2.0.2}

The same reprex with this PR draws them appropriately:

The implemented fix is easy: instead of taking the native labels as extracted from the scale, we use the validate_labels() function that handles expressions correctly.