fix(shortcodes): size the file fence to the file it wraps

[markdumay]

Summary

Fixes #2147.

The markdown variant of the file shortcode always opened a three-backtick fence:

{{- printf "```%s\n%s\n```" $lang (trim $content "\r\n") -}}

CommonMark ends a fenced block at the first closing run at least as long as the opening one. So when the file being embedded contains its own ```-fenced example, the emitted block closes partway through the file: the remainder is read as prose, and any later fence in the file re-opens and swallows what follows it.

This only affects the .md output — the HTML variant renders such a file correctly, so the two outputs disagree and only the markdown one is wrong.

Reproduce

A page with {{< file file="./skeleton.md" lang="markdown" >}} where skeleton.md is a documentation template that itself contains a ```text example. In the generated markdown the outer block ends at the inner example's closing fence rather than at the end of the file.

Fence structure before, for one such page:

  43  ```markdown     <- opens
 124  ```text
 131  ```             <- closes here (wrong)
 176  ```             <- re-opens

after:

  43  ````markdown    <- opens
 124  ```text
 131  ```             <- now just content
 176  ````            <- closes here (correct)

Approach

Scan the content for its longest fence run and open with one backtick more. A file with no fences is unaffected and still gets three.

Note

There is a second, independent bug behind the same symptom: mod-llm's safe-content.html matches fenced blocks with a single ```+ pattern, so a nested fence also ends its protected region early and the exposed tail is stripped from the output. Verified that fixing this shortcode alone does not resolve that — gethinode/mod-llm#147 covers it.

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[markdumay]

🎉 This PR is included in version 3.23.5 🎉

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