[Bug]: Spaced symbols should not be spaced when in limits etc

Bug summary

As seen, LaTeX does not add a space around the arrows (or =/+-sign) when in subscripts, but mathtext does.

Note that there is no space around \to but in mathtext,around \rightarrow in mathtext, while for LaTeX they look exactly the same. (This is because \to is not listed in _mathtext.Parser._arrow_symbols, but it should be.)

Code for reproduction

import matplotlib.pyplot as plt

strings = [r'$\limsup_{x\to\infty}$',
           r'$\limsup_{x\rightarrow\infty}$',
           r'$x \to \infty$',
           r'$x \rightarrow \infty$',
           r'$\limsup_{x=a+b}$',
           r'$x=a+b$',
           r'$A^{a \times b}_{c \Uparrow d}$']

for i, s in enumerate(strings):
    plt.text(0, i*0.15 + 0.05, s, fontsize=20)
    plt.text(0.5, i*0.15 + 0.05, f"${s}$", usetex=True, fontsize=20)

plt.axis(False)
plt.savefig("spacing.png", dpi=200)

Actual outcome

Expected outcome

No spaces around "spaced operators" in subscript.

Additional information

@devRD is this something you know how to fix?

Related to the failing test in https://github.com/matplotlib/matplotlib/pull/26173

Operating system

No response

Matplotlib Version

Latest main

Matplotlib Backend

No response

Python version

No response

Jupyter version

No response

Installation

None