Add header tests for Nexus operations (#1291)

Add tests verifying:
- Custom headers propagate from caller workflow to operation handler
- Outbound interceptors can add headers to Nexus operation requests
- Inbound interceptors can read and modify headers
- Headers work with @sync_operation, @workflow_run_operation, and custom OperationHandler
- Cancel operation headers are propagated and can be modified by interceptors

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Thomas Hardy <thestaffofmoses@gmail.com>

Author: 3 people