ActivityPub Handle Format Issue with Google Login

# ActivityPub Handle Format Issue with Google Login

## Description
When a user logs in via Google using **Site Kit by Google**, the automatically generated ActivityPub handle is formatted incorrectly.

Normally, a Fediverse handle should follow the format:

@username@domain.com

However, because the username is created using the full email address from the Google login, the resulting handle becomes improperly structured.

- **Expected Handle Format:** `@kimjiwoon75@travel-in-busan.com`
- **Actual Handle Format:** `@kimjiwoon75@gmail.com@travel-in-busan.com`

This issue affects the user's ability to be followed or interacted with on the Fediverse. The incorrect handle, which includes the `@gmail.com` part, is not a valid address.

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## Steps to Reproduce
1. Use Site Kit by Google to log in with a Google account (e.g., `kimjiwoon75@gmail.com`).
2. Install and activate the **ActivityPub** plugin.
3. Observe the generated ActivityPub handle.

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## Environment
- **WordPress Version:** [Insert your WordPress version]
- **Site Kit by Google Version:** [Insert your Site Kit version]
- **ActivityPub Plugin Version:** [Insert your ActivityPub version]
- **Profile URL:** [https://travel-in-busan.com/author/kimjiwoon75gmail-com/](https://travel-in-busan.com/author/kimjiwoon75gmail-com/)

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**Note:** To make the report as effective as possible, please include the version numbers for your WordPress, Site Kit, and ActivityPub plugins. This will help the developers identify and resolve the problem more quickly.

https://github.com/Automattic/wordpress-activitypub

No source file or test is named. Reproduce the Google login with Site Kit and the ActivityPub plugin, then trace where the generated author handle is assembled; done means the Google account produces a valid handle without the email provider suffix.

Written by the indexing model from the issue text.