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

[T4nya13]

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" } }
   }

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[Ch-saketh]

Hi team, I’ve reproduced this issue locally.

The problem seems to be that customFields provided in the users.update API call overwrite the existing custom fields instead of being merged. I’m working on a fix to apply a merge (patch) behavior so that only the specified keys are updated while preserving existing values.

I’ll open a PR once the fix is ready. Thanks!