### What problem does this PR solve?
- Use negative lookbehind (?<![a-zA-Z]) so \] and \) inside commands
(e.g. \right], \big)) are not treated as block/inline delimiters
- Use greedy matching to capture up to the last valid delimiter, fixing
truncated formulas (e.g. C_{seq}(y|x) = \frac{1}{|y|} ...)
- Add unit tests for preprocessLaTeX
Closes#13134
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
