Fix md mode comment (#3880)

* fix: fix a bug where md mode markdown breaks on comments

* chore: add changeset

.changeset/ten-frogs-mate.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix rendering HTML comments in `mode="md"` configuration

packages/astro/src/vite-plugin-markdown/index.ts

@@ -148,10 +148,12 @@ export default function markdown({ config }: AstroPluginOptions): Plugin {
 				// Turn HTML comments into JS comments while preventing nested `*/` sequences
 				// from ending the JS comment by injecting a zero-width space
 				// Inside code blocks, this is removed during renderMarkdown by the remark-escape plugin.
+				if (renderOpts.mode === 'mdx') {
 					markdownContent = markdownContent.replace(
 						/<\s*!--([^-->]*)(.*?)-->/gs,
 						(whole) => `{/*${whole.replace(/\*\//g, '*\u200b/')}*/}`
 					);
+				}
 
 				let renderResult = await renderMarkdown(markdownContent, {
 					...renderOpts,