The mail chatter now receives recipient field information as part of its standard mail data, avoiding extra background requests. This should make opening and updating chatter recipient information faster and reduce server calls without changing the visible workflow for users.

This PR removes `/mail/thread/recipients/fields` by embedding the value inside `/mail/data`. This removes up to 2 RPCs done inside chatter `onWillStart` and `updateRecipients`. 

PR enterprise: https://github.com/odoo/enterprise/pull/84661

Task-4685400

Forward-Port-Of: odoo/odoo#219345
Forward-Port-Of: odoo/odoo#208393

Web Studio was updated to stay compatible with recent changes in the messaging framework. This helps ensure form editor tests and related Studio behavior continue to work reliably after the underlying mail recipient fields endpoint was removed.

PR community: https://github.com/odoo/odoo/pull/208393 
Task-4685400

Forward-Port-Of: odoo/enterprise#90375
Forward-Port-Of: odoo/enterprise#84661