LaTeX: Diagnostics halt for the rest of the file after inline math ($...$) inside a macro

**Describe the bug**
`harper-ls` completely stops providing diagnostics for the remainder of a LaTeX document after encountering an inline math mode node (`$...$`) that is opened inside a macro (such as `\caption{`). After parsing the math node, the language server silently fails to check any further spelling or grammar errors.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a new `.tex` file with the following minimal working example:
```latex
\begin{document}
\section{TEST}
errawr % this typo will be caught by harper-ls

\begin{figure}
\caption{This is a test of the projection space ($N_{THC}$
\end{figure}

errawr % this typo will be completely ignored
\end{document}
```
2. Open the file in an editor with `harper-ls` attached.
3. Observe that diagnostics are provided for the first "errawr".
4. Scroll down past the `\caption` block and observe that the second "errawr" (and any subsequent text) receives no diagnostics.

**Expected behavior**
`harper-ls` should successfully parse and ignore the inline math block within the macro argument and continue providing diagnostics for the remainder of the document.

**Screenshots**
N/A - The issue can be reproduced strictly via the text snippet above.

**Platform**
Helix (via `harper-ls` v2.6.0).

**Additional context**
Through testing, the parser halt seems directly tied to how `harper-ls` handles the state transition of creating a math node while already inside a macro argument scope.
* The bug does NOT require the macro to be properly closed with `}`. The presence of the `$ ... $` pair inside the opening `{` is enough to trigger the bailout.
* If the dollar signs are escaped (`\$N_{THC}\$`), the server *does not* break and checks the whole file.
* If standard LaTeX parenthesis delimiters are used (`\( N_{THC} \)`), the server *does not* break.
The crash specifically occurs when paired `$` delimiters are successfully matched inside a macro, causing the AST parser to drop the rest of the file.

Start by reproducing the issue with the minimal .tex document and inspect the LaTeX AST parsing path for paired `$...$` math opened inside a macro argument. Done means the parser continues after that node and harper-ls reports diagnostics for the later "errawr" text, including when the macro is unclosed.

Written by the indexing model from the issue text.