[rush-lib] Add ensureFolderExists to plugin autoinstaller file writes

The read+write pattern introduced in PR microsoft#5678 replaced FileSystem.copyFile()
which implicitly created destination folders. The writeFile() calls need
ensureFolderExists: true to maintain the same behavior.

Author: kevin-y-ang

common/changes/@microsoft/rush-lib/fix-autoinstaller-ensure-folder_2026-03-12-00-00.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Fix AutoinstallerPluginLoader to use ensureFolderExists when writing plugin files",
+      "type": "patch",
+      "packageName": "@microsoft/rush-lib"
+    }
+  ],
+  "packageName": "@microsoft/rush-lib",
+  "email": "user@users.noreply.github.com"
+}

libraries/rush-lib/src/pluginFramework/PluginLoader/AutoinstallerPluginLoader.ts

@@ -73,7 +73,8 @@ export class AutoinstallerPluginLoader extends PluginLoaderBase<IRushPluginConfi
     // Use read+write instead of copy to ensure line endings are normalized
     const manifestContent: string = FileSystem.readFile(manifestPath);
     FileSystem.writeFile(destinationManifestPath, manifestContent, {
-      convertLineEndings: NewlineKind.Lf
+      convertLineEndings: NewlineKind.Lf,
+      ensureFolderExists: true
     });
     // Make permission consistent since it will be committed to Git
     FileSystem.changePosixModeBits(
@@ -105,7 +106,8 @@ export class AutoinstallerPluginLoader extends PluginLoaderBase<IRushPluginConfi
       // Use read+write instead of copy to ensure line endings are normalized
       const commandLineContent: string = FileSystem.readFile(commandLineJsonFullFilePath);
       FileSystem.writeFile(destinationCommandLineJsonFilePath, commandLineContent, {
-        convertLineEndings: NewlineKind.Lf
+        convertLineEndings: NewlineKind.Lf,
+        ensureFolderExists: true
       });
       // Make permission consistent since it will be committed to Git
       FileSystem.changePosixModeBits(