fix: strip trailing slashes from automatic GDCH audience (#8692)

Follow up PR to https://togithub.com/googleapis/google-cloud-node/pull/8301

Fixes https://togithub.com/googleapis/google-cloud-node/issues/8289 

See testing in [go/gemini-cli-sa](http://goto.google.com/gemini-cli-sa)

Author: jerrylin3321

core/packages/google-auth-library-nodejs/src/auth/googleauth.ts

@@ -1155,7 +1155,7 @@ export class GoogleAuth<T extends AuthClient = AuthClient> {
         const endpoint = opts.apiEndpoint || opts.servicePath;
         if (endpoint) {
           const scheme = endpoint.startsWith('http') ? '' : 'https://';
-          const formattedAudience = `${scheme}${endpoint}/`.replace(/\/+$/, '/');
+          const formattedAudience = `${scheme}${endpoint}`.replace(/\/+$/, '');
           const newClient = client.createWithGdchAudience(formattedAudience);
           this.cachedCredential = newClient;
           return newClient;

core/packages/google-auth-library-nodejs/test/test.googleauth.ts

@@ -3032,7 +3032,7 @@ describe('googleauth', () => {
       assert.ok(client instanceof GdchClient);
       assert.strictEqual(
         (client as GdchClient).apiAudience,
-        'https://hardwaremanagement.us-west1.gdch.google.com/'
+        'https://hardwaremanagement.us-west1.gdch.google.com'
       );
     });
 
@@ -3048,7 +3048,7 @@ describe('googleauth', () => {
       assert.ok(client instanceof GdchClient);
       assert.strictEqual(
         (client as GdchClient).apiAudience,
-        'https://hardwaremanagement.us-west1.gdch.google.com/'
+        'https://hardwaremanagement.us-west1.gdch.google.com'
       );
     });
 
@@ -3064,7 +3064,7 @@ describe('googleauth', () => {
       assert.ok(client instanceof GdchClient);
       assert.strictEqual(
         (client as GdchClient).apiAudience,
-        'http://hardwaremanagement.us-west1.gdch.google.com/'
+        'http://hardwaremanagement.us-west1.gdch.google.com'
       );
     });