Messaging and live chat avatars now use Odoo's standard image delivery path in most cases, reducing duplicate code and making behavior more consistent. This cleanup should make avatar handling easier to maintain while preserving the special live chat embed route needed for browser access rules.

This PR cleanup the extra avatar route by moving the access right check in
`_find_record_check_access`, allowing to always use the `web/image` route to get
avatars.
Custom route for im_livechat embed can't be removed for cors reasons.

This PR also uniformise the getter avatarUrl for thread and persona model,
allowing to delete the `threadservice.avatarUrl` function and `thread.imgUrl`
getter.

PR enterprise: https://github.com/odoo/enterprise/pull/52116

This update renames an internal WhatsApp contact image field from imgUrl to avatarUrl to align with related platform changes. It does not introduce new user-facing behavior, but helps keep the codebase consistent and easier to maintain.

See https://github.com/odoo/odoo/pull/142010

This change updates multiple Odoo Enterprise apps to use a newer internal way of communicating with the server. It should not change day-to-day behavior for users, but it helps keep the platform aligned with the latest core architecture and reduces obsolete code.