Adds a markdown_preview_code_font_family setting that overrides the font used in the markdown preview for code — inline code spans and fenced code blocks.
This is the counterpart to markdown_preview_font_family (#54003), which already does this for body text. Together they let you choose a typographically matched body + code font pair for the preview without forcing the code font onto editor buffers, where it may not be a good coding font:

{
  "markdown_preview_font_family": "Noto Serif",
  "markdown_preview_code_font_family": "Noto Sans Mono"
}

Behavior mirrors markdown_preview_font_family:

• Scoped to the markdown preview only (MarkdownFont::Preview). The agent panel, notifications, hover popovers, and REPL output are unaffected — they keep using the buffer font for code.
• Falls back to the buffer font family when unset, so existing previews are unchanged.
• Overrides the font family only; fallbacks and features still come from the buffer font.

Before (uses buffer font, here Iosevka):

After (uses specified font, here Noto Sans Mono):

Release Notes:

• Added markdown_preview_code_font_family to override the code font in the markdown preview