perf: reuse Google webhook OAuth client (#23361)

## Context
A new client currently downloads signing certificates for every webhook

## Fix
reuse same OAuth client instance

## Impact
Probably small but not really risky to merge imho

packages/twenty-server/src/modules/connected-account-sync-webhooks/drivers/google/__tests__/google-messaging-notification.handler.spec.ts

@@ -0,0 +1,62 @@
+import { OAuth2Client } from 'google-auth-library';
+
+import { GoogleMessagingNotificationHandler } from 'src/modules/connected-account-sync-webhooks/drivers/google/google-messaging-notification.handler';
+
+const mockVerifyIdToken = jest.fn();
+
+jest.mock('google-auth-library', () => ({
+  ...jest.requireActual('google-auth-library'),
+  OAuth2Client: jest.fn().mockImplementation(() => ({
+    verifyIdToken: (...args: unknown[]) => mockVerifyIdToken(...args),
+  })),
+}));
+
+describe('GoogleMessagingNotificationHandler', () => {
+  it('should reuse the OAuth client while verifying every notification', async () => {
+    mockVerifyIdToken.mockResolvedValue({
+      getPayload: () => ({
+        email: 'pubsub@example.com',
+        email_verified: true,
+      }),
+    });
+
+    const handler = new GoogleMessagingNotificationHandler(
+      {
+        get: jest.fn((key: string) => {
+          if (key === 'MESSAGING_GMAIL_PUBSUB_VERIFICATION_EMAIL') {
+            return 'pubsub@example.com';
+          }
+
+          if (key === 'SERVER_URL') {
+            return 'https://example.com';
+          }
+        }),
+      } as never,
+      {
+        incrementCounterBy: jest.fn(),
+      } as never,
+      {} as never,
+      {} as never,
+      {} as never,
+    );
+
+    await handler.handle({
+      authorizationHeader: 'Bearer first-token',
+      body: {},
+    });
+    await handler.handle({
+      authorizationHeader: 'Bearer second-token',
+      body: {},
+    });
+
+    expect(OAuth2Client).toHaveBeenCalledTimes(1);
+    expect(mockVerifyIdToken).toHaveBeenNthCalledWith(1, {
+      idToken: 'first-token',
+      audience: 'https://example.com/webhooks/google/messaging',
+    });
+    expect(mockVerifyIdToken).toHaveBeenNthCalledWith(2, {
+      idToken: 'second-token',
+      audience: 'https://example.com/webhooks/google/messaging',
+    });
+  });
+});

packages/twenty-server/src/modules/connected-account-sync-webhooks/drivers/google/google-messaging-notification.handler.ts

@@ -34,6 +34,7 @@ export type GoogleMessagingNotificationRequest = {
 @Injectable()
 export class GoogleMessagingNotificationHandler implements WebhookNotificationHandler<GoogleMessagingNotificationRequest> {
   private readonly logger = new Logger(GoogleMessagingNotificationHandler.name);
+  private readonly oauth2Client = new OAuth2Client();
 
   constructor(
     private readonly twentyConfigService: TwentyConfigService,
@@ -125,7 +126,7 @@ export class GoogleMessagingNotificationHandler implements WebhookNotificationHa
     const expectedAudience = `${this.twentyConfigService.get('SERVER_URL')}/webhooks/google/messaging`;
 
     try {
-      const ticket = await new OAuth2Client().verifyIdToken({
+      const ticket = await this.oauth2Client.verifyIdToken({
         idToken,
         audience: expectedAudience,
       });