[Repo Assist] test+fix: add Markdown.ToLatex unit tests; fix level-6 heading bug

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🤖 This is an automated pull request from Repo Assist.

Summary

Two related improvements to Markdown.ToLatex:

1. Testing (Task 9): Add 29 direct unit tests for Markdown.ToLatex — this output format previously had zero unit test coverage (only obsolete file-comparison tests in TestFiles.fs and two indirect Literate tests).

2. Bug fix (Task 3): Fix a bug where level-6 headings (######) produced invalid LaTeX output.

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Bug Fix: Level-6 Headings in ToLatex

Root cause: LatexFormatting.fs matched heading levels 1–5 to LaTeX commands, but had | _ -> "" as the catch-all. For a level-6 heading, level = "", producing {content} — braces without any command prefix, which is invalid LaTeX.

Fix: Replace | _ -> "" with | _ -> @"\subparagraph". Since \subparagraph is the deepest LaTeX sectioning command, all headings at level 5 and above correctly map to it.

Example:

Before: {Level Six}   ← invalid LaTeX
After:  \subparagraph{Level Six}   ← valid

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New Test Coverage

29 tests added to tests/FSharp.Markdown.Tests/Markdown.fs covering:

Feature	LaTeX command
Heading level 1	\section*{...}
Heading level 2	\subsection*{...}
Heading level 3	\subsubsection*{...}
Heading level 4	\paragraph{...}
Heading level 5	\subparagraph{...}
Heading level 6 (new fix)	\subparagraph{...}
Bold	\textbf{...}
Italic	\emph{...}
Inline code	\texttt{...}
Link	\href{url}{text}
Unordered list	\begin{itemize}...\item...\end{itemize}
Ordered list	\begin{enumerate}...\item...\end{enumerate}
Code block	\begin{lstlisting}...\end{lstlisting}
Blockquote	\begin{quote}...\end{quote}
Horizontal rule	\noindent\makebox...
Table	\begin{tabular} with bold headers
Image with alt	\begin{figure}...\caption{...}...\end{figure}
Image without alt	\includegraphics{...}
Special char escaping	\#, \%, \&, \_
Inline math	$E = mc^2$
Display math ($$)	\begin{equation}...\end{equation}
EmbedParagraphs	delegates to Render()
Empty document	no error

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Test Status

• 346 markdown tests pass (up from 317 before this PR)
• 143 literate tests pass — no regressions
• Build: 0 warnings, 0 errors

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