Fix LaTeXWriter not stripping $$ delimiters from text/latex MIME output

[singhharsh1708]

When a type's show(io, ::MIME"text/latex", ...) wraps output in $$
or \[...\] around a display math environment (e.g. ModelingToolkit),
LaTeXWriter was writing the content verbatim. This caused a 'Bad math
environment delimiter' LaTeX error because both $$ and \begin{equation}
try to open math mode.
Add _strip_latex_math_delimiters which strips outer $$/\[...\] delimiters
only when the inner content already contains a \begin{...} environment.
Plain math content like $$ \left[...\right] $$ is left unchanged.
Fixes #2923

[odow]

I maintain that this is a bug in ModelingToolkit:

$$ \begin{equation}
\frac{dv(t)}{dt} = g
\end{equation}
$$

That's not valid LaTeX.

[singhharsh1708]

You're right that $$\begin{equation}...\end{equation}$$ is invalid LaTeX — the bug belongs in the package generating it.
I've updated the approach: Documenter now emits a @warn when it detects this pattern, explicitly telling users to fix the show(io, MIME"text/latex"(), ...) method of the relevant type. The outer delimiters are still stripped so the PDF build doesn't crash with an opaque "Bad math environment delimiter" error, but the warning makes the real problem visible rather than silently papering over it.
Happy to close this if the preference is to let it fail loudly instead.

[odow]

I don't think there needs to be any change in Documenter. We shouldn't try to correct other people's incorrect LaTeX.

[odow]

I assume the issue is here:

https://github.com/SciML/ModelingToolkit.jl/blob/8d382c8e10c9cfa77807bcc2716074e5635f9470/lib/ModelingToolkitBase/ext/MTKLatexifyExt.jl#L21-L23

[asinghvi17]

It's probably because latexify emits \begin{equation}...\end{equation} by default - https://github.com/korsbo/Latexify.jl/blob/8b633ca3bbadf5e85b27b57555a76574cc15e786/src/latexequation.jl#L4

Ideally there would be some way to tell it not to - which SciML/MTK could use. But at a brief glance I could not find one.

[singhharsh1708]

Understood, closing this. The fix belongs upstream — either in Latexify.jl to provide a way to suppress the \begin{equation} wrapper, or in ModelingToolkit's MTKLatexifyExt.jl to use that option once available.