fix(access-control): populate GA4 "group" metadata for anonymous readers

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Follow-up to #4697 and #4746. Allows the GA4 group param to populate for non-logged-in readers who match an institution via IP access rules.

How to test the changes in this Pull Request:

Prerequisites:

• NEWSPACK_CONTENT_GATES defined in wp-config.php
• WooCommerce Memberships plugin deactivated
• A published Institution (Audience > Access Control > [kebab menu] Institutions) with an IP whitelist access rule
• Google Site Kit connected with Google Analytics enabled

1. In an incognito session, without registering or logging in, visit the institution's access URL using an IP that falls in the institution's whitelisted IP range and wait until you get redirected to the homepage with the "Connected to..." success message.
2. View the page source and confirm that you see the "group": "Institution #####" param appended to the gtag("config") call.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

This PR updates the Google Site Kit GA4 integration so the GA4 group custom parameter can be populated for anonymous visitors who match an Institution via IP-based access rules (e.g., after passing /institutional-access and receiving the cache-bypass cookie).

Changes:

• Pre-populate GA4 group labels with matching Institution IDs even when the current user is not logged in.
• Adjust early-return behavior in get_user_group_labels() so anonymous users can still receive Institution labels.

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[github-actions]

🎉 This PR is included in version 6.41.3-hotfix-group-metadata-for-anon-readers.2 🎉

The release is available on GitHub release

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