ADM-OPERATOR-USER-203 - Update Operator User

PUT https://example.com/v1/operator-users/{id}

Overview
Updates an operator user identified by id, including optional group assignment and store restriction.

User scoping
Admin endpoint. Only operator users can call this endpoint (dj-client: OPERATOR).

Store scoping
dj-store and dj-store-view define the request context. The target operator user can be restricted to specific stores using stores.

Validation rules
If stores is provided, it must be an array of store external identifiers (externalId); duplicates are ignored.

Business rules
Replacement rules for stores:
- If omitted: the current store restriction is preserved.
- If provided as an empty array: removes any restriction (allowed on all stores).
- If provided with values: replaces the current restriction.

Errors
Returns validation errors (400), authentication errors (401), authorization errors (403), not found (404), and unexpected server errors (500).