ADM-OPERATOR-USER-350 - Remove Operator User Stores

Overview
Removes stores from the operator user identified by operatorUserId.

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

Store scoping
The effective store context is derived from the dj-store and dj-store-view headers when provided. This context does not impact store assignment.

Removal rules
- Stores in the request are removed from the current assignment.
- Non-assigned stores in the request are ignored.
- Empty array in request body is a no-op.

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

Language
Credentials
Bearer
JWT
Response
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