Revert "Make MDX integration check noop (#12913)"

.changeset/calm-tips-think.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/mdx': patch
+---
+
+Reverts /~https://github.com/withastro/astro/commit/9a3b48c5c3e8f597159454f06c5a0ce8e709bc50 which caused a regression for rendering inline MDX components and MDX files from content collections

packages/integrations/mdx/src/server.ts

@@ -1,16 +1,29 @@
 import type { NamedSSRLoadedRendererValue } from 'astro';
 import { AstroError } from 'astro/errors';
-import { jsx } from 'astro/jsx-runtime';
+import { AstroJSX, jsx } from 'astro/jsx-runtime';
 import { renderJSX } from 'astro/runtime/server/index.js';
 
 const slotName = (str: string) => str.trim().replace(/[-_]([a-z])/g, (_, w) => w.toUpperCase());
 
-// MDX components are tagged with `__astro_tag_component__` in `vite-plugin-mdx-postprocess.ts`
-// and tagged componenets can directly tell Astro which renderer to use in build-time, rather
-// than checking the components in runtime. So this function is made a no-op as any untagged
-// component wouldn't have belonged to MDX.
-export async function check() {
-	return false;
+// NOTE: In practice, MDX components are always tagged with `__astro_tag_component__`, so the right renderer
+// is used directly, and this check is not often used to return true.
+export async function check(
+	Component: any,
+	props: any,
+	{ default: children = null, ...slotted } = {},
+) {
+	if (typeof Component !== 'function') return false;
+	const slots: Record<string, any> = {};
+	for (const [key, value] of Object.entries(slotted)) {
+		const name = slotName(key);
+		slots[name] = value;
+	}
+	try {
+		const result = await Component({ ...props, ...slots, children });
+		return result[AstroJSX];
+	} catch (e) {
+		throwEnhancedErrorIfMdxComponent(e as Error, Component);
+	}
 }
 
 export async function renderToStaticMarkup(