Use Kit Account ID as WPForms Provider Key

[n7studios]

Summary

When completing OAuth, credentials are stored in WPForms using wpforms_update_providers_options, with a random ID as the key.

If a user disconnects and then connects the same Kit account (most likely as a workaround for the access token issue in the v4 API), this results in a different random ID as the key for the stored credentials.

As a result, any configuration stored in Forms is lost, as WPForms maps configurations to the keys in wpforms_update_providers_options.

This PR resolves by using the Kit account ID as the key when storing credentials, ensuring that any configuration at Form level remains mapped to the correct Kit account, without any loss of configuration and saving users having to reconfigure the Kit integration on each WPForms Form.

This doesn't resolve the issue of the same access and refresh token issuance to multiple sites using the same Kit account, which is covered by PLAT-2262 - rather, it is a way to reduce friction should a creator need to disconnect and connect a Kit account manually for the time being.

Testing

• testConnectionRetainedWhenAccountReconnected: Tests that an existing Kit configuration on a WPForms Form is retained when an OAuth connection is added, removed and added again.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)