Replaces an existing user resource via the SCIM 2.0 PUT operation (RFC 7644 §3.5.1). The request body represents the full desired state of the resource. All required attributes must be provided. Unsupported or read-only attributes (e.g. id, meta) in the request body are ignored.
Note: This implementation deviates from strict RFC 7644 PUT semantics for externalId — when externalId is absent from the request, the existing value is preserved rather than cleared. To update externalId, include it explicitly. Setting externalId to null when a value already exists returns 400 (mutability).
When active is absent from the request, it preserves the existing value.