Attach X-ILM-Attribute header on Attributes v2 callback dispatch

[lubomirw]

Description

Core dispatches NG attribute callbacks to a connector's single POST /v2/attributes/callback endpoint. To keep per-callback visibility in connector access logs despite the single endpoint, Core should attach a non-binding X-ILM-Attribute: <attributeName> header on the dispatch (mirrors the authority-v3 X-ILM-Operation convention).

This is currently not implemented — verified absent from both the Core callback dispatch and the interfaces clients/v2 Attributes client. Split out of the NG callback dispatch work (core #1622 / PR #1679) as a non-blocking observability follow-up.

Definition of Done

• The X-ILM-Attribute header carrying the attribute name is set on POST /v2/attributes/callback for both HTTP and MQ transports.
• A test asserts the header is present on dispatch.
• Non-binding: the connector is not required to read it (no contract change).