Support Google Account OAuth and reuse of CLI authentication file

[synote]

Currently the antigravity-sdk-python only supports API Key authentication. This limitation presents two main problems:

• Google One & Advanced Subscribers: Many individual developers have consumer premium subscriptions, they cannot use their personal Google accounts to authenticate within the SDK and are instead forced to set up developer API keys.
• Authentication Fragmentation: if a developer has already logged in and authenticated antigravity-cli, they still have to manually create and export an API key to use the SDK.

Proposal

I recommend supporting Google Account OAuth Authentication natively in the Python SDK. The solution should ideally provide:

• Direct Integration with CLI Credentials: The SDK connection strategies should automatically check for existing credentials created by antigravity-cli and use the auth json to proceed authorize request.
• Authentication Methods on Initialization: Provide a clear authentication flow within the SDK to trigger browser-based Google OAuth and return redirect url

[synote]

We are currently planning to migrate GeminiCLI in Sublime Text to use the antigravity backend/SDK. Users usually manage their own authentication locally in the CLI and A significant portion uses Google One subscribe.
Integrating antigravity credential would naturally provide a zero-configuration developer experience for anyone who has already run the CLI's login command:

[Unravl]

This needs to happen. I want to implement this SDK but not without the ability to use the subscriptions.