[Bug] users.update endpoint overwrites existing customFields instead of merging

Description

When updating a user's profile via the users.update API endpoint, providing a customFields object causes a complete overwrite of the user's existing custom fields. This results in unintended data loss for any fields not explicitly included in the payload.

The expected behavior is a merge (patch) operation, where only the specified keys are updated, and existing keys remain untouched.

Steps to reproduce

1. Enable Custom Fields in Administration -> Accounts -> Registration.
2. Create two custom fields (e.g., role and team).
3. Set initial values for a user: { "role": "Developer", "team": "Red" }.
4. Send a POST request to api/v1/users.update with the following payload (updating only one field):

   {
     "userId": "TARGET_USER_ID",
     "data": { "customFields": { "team": "Blue" } }
   }

Start at the users.update API endpoint and reproduce the issue using the payload in the report, with existing customFields for role and team. The fix is complete when updating team preserves role and changes only the submitted custom-field keys; verify the behavior through the endpoint reproduction.