Enable Swift Package Manager before Flutter pub get on ci

flutter config --enable-swift-package-manager must run after Flutter
is installed and before flutter pub get so that SPM-based dependencies
(Auth0.swift resolved via Package.swift) are properly set up when
building and running native iOS/macOS unit tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: NandanPrabhu

.github/actions/setup-darwin/action.yml

@@ -50,6 +50,10 @@ runs:
         channel: stable
         cache: true
 
+    - name: Enable Swift Package Manager
+      run: flutter config --enable-swift-package-manager
+      shell: bash
+
     - name: Install Flutter dependencies
       working-directory: auth0_flutter/example
       run: flutter pub get