OAuth login fails for Google AI Pro users: "This client is no longer supported for Gemini Code Assist for individuals"

### What happened?

## Summary

Gemini CLI fails to authenticate using Google Sign-In even though browser authentication succeeds.

## Environment

- OS: Windows 11
- Shell: PowerShell
- Gemini CLI: v0.49.0
- Installation:
npm install -g @google/gemini-cli
- Subscription:
Google AI Pro

## Steps to Reproduce

1. Install Gemini CLI.
2. Run `gemini`.
3. Select "Sign in with Google".
4. Browser opens.
5. Sign in with a Google AI Pro account.
6. Browser displays "Authentication successful".
7. Return to the terminal.

## Expected Behavior

Gemini CLI should authenticate successfully and start normally.

## Actual Behavior

Authentication fails with:

Failed to sign in.

Message:
This client is no longer supported for Gemini Code Assist for individuals.
To continue using Gemini, please migrate to the Antigravity suite of products.

## Additional Information

- Reinstalled Gemini CLI.
- Deleted the ~/.gemini configuration.
- No GEMINI_API_KEY environment variables are present.
- Browser authentication completes successfully.
- Problem persists on a clean installation.

### What did you expect to happen?

I expected Gemini CLI to successfully authenticate my Google AI Pro account after I selected "Sign in with Google". After completing the OAuth login in my browser and seeing the "Authentication successful" page, I expected the CLI to return to the terminal, complete the authentication process, and start an interactive Gemini session without any errors.

### Client information

Client Information

Run `gemini` to enter the interactive CLI, then run the `/about` command.

```console
> /about
# paste output here
```

### Login information

_No response_

### Anything else we need to know?

_No response_

No source file or test is named in the issue. Start by reproducing the Google Sign-In flow from `gemini` on Windows PowerShell with a Google AI Pro account, then use `/about` to record client information. Done means the browser callback completes, the terminal session starts normally, and the unsupported-client error no longer appears.

Written by the indexing model from the issue text.