Fix the stability of UT decoding and add extra protection to downstream tracking
The downstream validator remained unstable after !1739 (merged), but this MR has definitively fixed the issue.
A local test was performed by running hlt1_pp_forward_then_matching_and_downstream_validation 100 times on an RTX A5000 and checking for differences in its output. This MR passed the test without any exceptions.
The fix originated from the discussion at !1768 (comment 8484715).
The solution involves replacing the tiebreak part of the sorting key with LHCbID.
Additionally, this MR adds extra protection to downstream tracking. Although we haven't reached this limit yet, it's good to have this safeguard in place.