fix: Skip externals doc validation when doc files don't exist

The validate-externals-doc.js script was checking if the docs/ directory
exists but not if the specific serverExternalPackages.mdx files exist.
This caused lint failures in forks that have an empty docs/ directory.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: celicoo

scripts/validate-externals-doc.js

@@ -11,10 +11,15 @@ if (!fs.existsSync(docsDir)) {
 }
 
 function validate(docPath) {
-  const docContent = fs.readFileSync(
-    path.join(__dirname, '..', docPath),
-    'utf8'
-  )
+  const fullPath = path.join(__dirname, '..', docPath)
+
+  // Skip validation if the specific doc file doesn't exist (e.g., in forks without full docs)
+  if (!fs.existsSync(fullPath)) {
+    console.log(`doc file not found: ${docPath}, skipping validation`)
+    return true
+  }
+
+  const docContent = fs.readFileSync(fullPath, 'utf8')
 
   const docPkgs = []
   const extraPkgs = []