fix(google-signin): return tokenString instead of GIDToken object on iOS

[przemyslawturek]

Description

Fix iOS token retrieval returning GIDToken object instead of string value.

Problem

On iOS, getTokens() returns native GIDToken objects instead of token strings, making the tokens unusable in JavaScript.

Solution

Use .tokenString property from GIDToken objects to extract actual token values.

Testing

• [x] Tested on iOS device iOS 16.7.12 and simulator iOS 17.5 also iOS 18.0
• [x] Verified tokens are now strings
• [x] Sign-in flow works correctly sending token to BE

Platforms affected

• [x] iOS
• [ ] Android