DPL: move away from MessageSet::header / payload

[ktf]

Abstract header / payload retrieval, with the idea that get_header / get_payload
will work on any range of fair::mq::MessagePtrs.

For now we only do the first header / payload pair only, to validate the trivial change.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 54a490d at 2026-03-24 12:04:

## sw/BUILD/O2-full-system-test-latest/log
command /sw/slc9_x86-64/O2/15208-slc9_x86-64-local1/prodtests/full-system-test/dpl-workflow.sh had nonzero exit code 1
[10335:gpu-reconstruction]: [12:04:26][FATAL] error while setting up workflow in o2-gpu-reco-workflow: Payload not found
[ERROR] Workflow crashed - PID 10335 (gpu-reconstruction) did not exit correctly however it's not clear why. Exit code forced to 128.
[ERROR] Workflow crashed - PID 10475 (tpc-miptrack-filter) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10447 (mch-track-finder) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10653 (TRD-Tracking-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10485 (Dispatcher) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10639 (CPV-PhysicsOnEPNs-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10649 (TRD-PHTrackMatch-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10646 (MFT-MFTAsyncTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10465 (secondary-vertexing) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10647 (TPC-PID-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10689 (TRD-Digits-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10680 (TOF-MatchingTOFwTRD-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10522 (qc-task-EMC-RawTask) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10663 (GLO-MTCITSTPC-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10518 (zdc-tdccalib) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10459 (muon-track-matcher) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10460 (globalfwd-track-matcher) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10674 (MID-QcTaskMIDDigits-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10457 (mch-error-merger) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10692 (ZDC-QcZDCRecTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10682 (TPC-Clusters-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10662 (FV0-DigitQcTaskFV0-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10667 (ITS-ITSClusterTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10685 (TPC-Tracks-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10672 (MFT-MFTClusterTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10435 (primary-vertexing) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10504 (tpc-calibrator-dEdx) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10665 (GLO-Vertexing-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10487 (residual-aggregator) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10681 (TOF-TaskDigits-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10644 (GLO-MUONTracks-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10500 (tof-merge-integrated-clusters) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10453 (mean-vertex-calibration) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10669 (ITS-ITSTrackTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10676 (MID-QcTaskMIDTracks-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10657 (EMC-RawTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10658 (FDD-DigitQcTaskFDD-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10670 (MCH-QcTaskMCHDigits-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10673 (MID-QcTaskMIDClust-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10463 (pvertex-track-matching) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10661 (FT0-DigitQcTaskFT0-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10479 (o2-eve-export) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10596 (qc-task-TPC-Clusters) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10679 (PHS-ClusterTask-proxy) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10495 (ft0-time-offset-calib) was killed abnormally with Killed and exited code was set to 137.
[ERROR] Workflow crashed - PID 10554 (qc-task-MCH-QcTaskMCHDigits) was killed abnormally with Killed and exited code was set to 137.
[0 more errors; see full log]

Full log here.