CI coverage flagged MarkdownText's pre route: the array-element probe
(raw[0]) and the mixed-content fallbacks were unreachable — the markdown
pipeline hands pre one code element whose children are one string (or
none, for an empty fence). Simplify to the string check, annotate the
isValidElement guard as representation-change armor, and pin both live
arms: the empty fence keeps the stock <pre>, a language-less fence renders
the plain CodeBlock arm.
Tianyi Cui
28b617dd73