fix(next-mdx): resolve ESM-only plugins passed as strings

When an MDX plugin is passed as a string (e.g.
`rehypePlugins: ['@stefanprobst/rehype-extract-toc']`), the loader
resolves it via `require.resolve()` which uses CJS resolution
semantics.  Packages that ship with an `exports` field containing only
an `"import"` condition (no `"require"` condition) cause
`require.resolve` to throw ERR_PACKAGE_PATH_NOT_EXPORTED because no
CJS-compatible export entry is found.

Catch that specific error and fall back to a direct ESM dynamic
`import()`, which uses the ESM resolver and correctly matches the
`"import"` condition in the exports map.

Fixes #73757

Author: 20syldev

packages/next-mdx/mdx-js-loader.js

@@ -1,17 +1,73 @@
 const mdxLoader = require('@mdx-js/loader')
+const fs = require('node:fs')
+const path = require('node:path')
 const { pathToFileURL } = require('node:url')
 
 function interopDefault(mod) {
   return mod.default || mod
 }
 
+// Recursively resolve an exports condition value, preferring 'import' then 'default'.
+function resolveExportCondition(value) {
+  if (typeof value === 'string') return value
+  if (value && typeof value === 'object') {
+    for (const key of ['import', 'default']) {
+      if (key in value) {
+        const result = resolveExportCondition(value[key])
+        if (result) return result
+      }
+    }
+  }
+  return null
+}
+
+// Extract the ESM entry point from a package.json object.
+function resolveEsmEntry(pkg) {
+  const { exports: exp, module: mod, main } = pkg
+  if (exp) {
+    const mainExport = typeof exp === 'string' ? exp : exp['.'] ?? exp
+    const resolved = resolveExportCondition(mainExport)
+    if (resolved) return resolved
+  }
+  return mod ?? main ?? 'index.js'
+}
+
 async function importPluginForPath(pluginPath, projectRoot) {
-  const path = require.resolve(pluginPath, { paths: [projectRoot] })
+  let resolvedPath
+  try {
+    resolvedPath = require.resolve(pluginPath, { paths: [projectRoot] })
+  } catch (e) {
+    if (e.code === 'ERR_PACKAGE_PATH_NOT_EXPORTED') {
+      // The package defines an `exports` field but has no entry for the
+      // `"require"` condition (e.g. an ESM-only package whose exports only
+      // include an `"import"` condition). `require.resolve` uses CJS
+      // resolution semantics and cannot match those entries.
+      //
+      // Node.js always includes the absolute path to the package's package.json
+      // in the error message. That path was resolved starting from `projectRoot`,
+      // so we can use it to find the ESM entry point and import via a file:// URL.
+      // A bare import(pluginPath) would resolve from @next/mdx's own location,
+      // which fails in pnpm / non-hoisted setups.
+      const pkgJsonMatch = e.message.match(/in (.+package\.json)/)
+      if (pkgJsonMatch) {
+        const pkgJsonPath = pkgJsonMatch[1]
+        const pkgDir = path.dirname(pkgJsonPath)
+        const pkg = JSON.parse(fs.readFileSync(pkgJsonPath, 'utf8'))
+        const entry = resolveEsmEntry(pkg)
+        const absolutePath = path.resolve(pkgDir, entry)
+        return interopDefault(await import(pathToFileURL(absolutePath)))
+      }
+      // Could not extract the path from the error message — fall back to a bare
+      // import (may fail in strict pnpm / non-hoisted setups).
+      return interopDefault(await import(pluginPath))
+    }
+    throw e
+  }
   return interopDefault(
     // "use pathToFileUrl to make esm import()s work with absolute windows paths":
     // on windows import("C:\\path\\to\\file") is not valid, so we need to use file:// URLs
     // https://github.com/vercel/next.js/commit/fbf9e12de095e0237d4ba4aa6139d9757bd20be9
-    await import(process.platform === 'win32' ? pathToFileURL(path) : path)
+    await import(process.platform === 'win32' ? pathToFileURL(resolvedPath) : resolvedPath)
   )
 }