Add headers to cancel nexus task

[Quinn-With-Two-Ns]

Add headers to cancel nexus task. I do have a test we can add along side this, it is decently long for how simple this change is so I left it off for now, but if we want I can add it.

closes #2797

---

Note

Low Risk
Single-field addition to the cancel Nexus task request; low chance of regressions beyond minor behavior changes in tests that assert exact request contents.

Overview
Propagates the original Nexus headers when the test server enqueues a CancelOperationRequest for an in-flight Nexus operation, by copying NexusOperationScheduledEventAttributes.nexus_header into the cancel Request.

This ensures cancel tasks carry the same header context as start tasks (e.g., for auth/metadata parity) rather than sending an empty header map.

^{Written by Cursor Bugbot for commit 3d98352. This will update automatically on new commits. Configure here.}