[FIX] mail: fix message create returning proxified version

Double assignation does not chain, resulting in the non proxified
message being returned, which can potentially cause issues either when
comparing messages by reference or if a reactive version is expected.

X-original-commit: 30a77cb2778ec0fce5892cfd3903aa1982825626
Part-of: odoo/odoo#125439

addons/mail/static/src/core/common/message_service.js

@@ -218,7 +218,8 @@ export class MessageService {
         } else {
             message = new Message();
             message._store = this.store;
-            message = this.store.messages[data.id] = message;
+            this.store.messages[data.id] = message;
+            message = this.store.messages[data.id];
         }
         this.update(message, data);
         // return reactive version