Add background script for OAuth token handling example

This script demonstrates how to use the OAuthClientCredsHelper for automatic OAuth bearer token handling in API calls.

Author: hanna-g-sn

Integration/RESTMessageV2/Auth2 client credentials token cache with auto-refresh/example_background_usage.js

@@ -0,0 +1,23 @@
+// Background Script example: call an API with automatic OAuth bearer handling
+(function() {
+  var helper = new OAuthClientCredsHelper();
+
+  var options = {
+    // Token settings
+    tokenUrl: 'https://auth.example.com/oauth2/token',
+    clientId: 'YOUR_CLIENT_ID',
+    clientSecret: 'YOUR_CLIENT_SECRET', // store securely in real environments
+    scope: 'read:things',               // optional
+    audience: '',                       // optional (for some IdPs)
+    propPrefix: 'x_acme.oauth.sample',  // system property prefix for cache
+
+    // Resource request
+    resource: 'https://api.example.com/v1/things?limit=25',
+    method: 'GET',
+    headers: { 'Accept': 'application/json' }
+  };
+
+  var res = helper.request(options);
+  gs.info('Status: ' + res.status + ', refreshed=' + res.refreshed);
+  gs.info('Body: ' + res.body);
+})();