Fix mathtext spacing of operators at expression boundaries - #32331

PR summary

Closes #23315.

Relation, binary and arrow operators are "spaced symbols", mathtext puts a space on both sides of them. But TeX drops the glue at the boundaries of a math list, so an operator at the very start or end of an expression (or of a {...} group) shouldn't be spaced on that side. For example, in a$=b$ there's no space before the =, only after it. mathtext spaced both sides, and only binary operators were special-cased, and only at the start of the string.

I now decide the left and right spacing of each spaced symbol independently, and drop the space on any side that touches the start/end of the expression or an opening/closing delimiter. I also fixed the next-character lookahead so it skips past the whole source token (e.g. a multi-character command like \doteq) instead of just loc + 1, which is what made named relations at the end still get a stray space.

This lets me restore the simpler \ddots test that previously needed a negative \hspace workaround, and I added a check_figures_equal regression test.

Two existing image tests change, and I've regenerated their baselines (built locally with matplotlib's bundled FreeType, the same one the image tests use):

• test_mathtext_rendering index 71 ($\Cap$ $\Cup$ $\leftharpoonup$ $\barwedge$ $\rightharpoonup$): the harpoons, alone in $...$, no longer get edge spacing.
• test_mathtext_rendering_lightweight index 1: the \rightarrow/\leftarrow inside \overset/\underset no longer get edge spacing.

Both are correct changes. I verified index 72 (the \ddots test) renders pixel-identically to its existing baseline across all fontsets, so it does not need regeneration.

AI Disclosure

I used Claude Code to explore and navigate the mathtext parser and scaffold the regression test. I decided on the approach, edited the code and made the changes. Then, I checked the result against TeX's spacing: compared rendered output before and after the change and confirmed the restored \ddots test matches its existing baseline.

• Use an expressive title
• New and changed code is tested: a new check_figures_equal test and the restored \ddots test, plus regenerated baselines for the two image tests whose rendering changes.
• N/A: no new plotting feature to demonstrate in an example.
• Behavior change documented in doc/api/next_api_changes/behavior/23315-AM.rst.
• N/A: no other documentation changes needed.