fix: LaTeX diagnostics halt after unmatched inline math ($) inside a macro

[mvanhorn]

Issues

Fixes #3791

Description

The tex masker lives in harper-tex/src/masker.rs. Its top-level create_mask loop reaches \caption{...} and calls deconstruct_command, whose curly-content scan stops at the FIRST } (the inner } of N_{THC}), truncating the caption argument so the cursor lands on the second, now-unpaired $. The loop then calls math_mode_at_cursor on that lone $; because there is no closing $ left in the document it counts every remaining character and returns remaining + 2, so the following PushMaskAndIncBy masks the entire rest of the file - dropping all later tokens and hence all diagnostics. Primary fix: make math_mode_at_cursor require a matching closing $ - when no closing $ follows the opener, return None (or mask only the single $) instead of consuming to end-of-file, mirroring the running-off-the-end guard already present in equation_at_cursor. This restores diagnostics for the remainder of the document exactly as the reporter expects, and is a minimal, convergent change. The deeper root cause (naive first-} curly-argument parsing in deconstruct_command not balancing nested braces) may optionally be hardened in the same function, but the reported halt is fully resolved by the math_mode_at_cursor guard, so scope stays tight to avoid regressing existing tex snapshots.

Demo

N/A - behavioral fix; validated by the tests below.

How Has This Been Tested?

• Happy path: add harper-tex/tests/test_sources/issue_3791.tex reproducing the issue snippet (two errawr typos around a \caption{ ... ($N_{THC}$ block) and a create_test!(issue_3791.tex, N) entry in run_tests.rs asserting both typos are still linted (rest-of-file diagnostics no longer suppressed), following the existing issue_2835.tex precedent.
• Unit test in the #[cfg(test)] mod tests block of masker.rs: a lone/unmatched $ does not mask the remainder - text after the stray $ remains in the allowed spans of the mask.
• Edge cases: a correctly paired top-level $...$ is still fully masked; \$ ... \$ and \( ... \) continue to work; empty $$ and a trailing $ at end-of-file neither panic nor over-consume.
• Error path: create_mask does not panic on a document that ends with an unclosed $ (extends the existing trailing-backslash non-panic guarantees).

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

If Your PR Implements or Enhances a Linter

• I made up the sentences in the unit tests.
• The sentences in the unit tests were generated by an AI.
• I'm using examples from the bug report / feature request.
• I collected real-world sentences for the unit tests.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.