[FIX] mail: assign localId directly on record

There is no need to go through the proxy to assign localId as nothing
should observe it at this step, and it shouldn't change anyway.

Part-of: odoo/odoo#147814

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

@@ -1429,10 +1429,8 @@ export class Record {
                     );
                 }
             }
-            Object.assign(recordProxy, {
-                localId: Model.localId(ids),
-                ...ids,
-            });
+            Object.assign(record, { localId: Model.localId(ids) });
+            Object.assign(recordProxy, { ...ids });
             ModelProxy.records[record.localId] = recordProxy;
             if (record._RawModel.name === "Store") {
                 Object.assign(record, {