Render fenced code blocks with Gitea's own code block renderer instead
of `goldmark-highlighting`, using the managed lexer lookup from
`modules/highlight`. Indented and fenced blocks now share one rendering
path.
Rendered output is unchanged. The trailing newline is stripped on copy
in the frontend since https://github.com/go-gitea/gitea/pull/39755.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>