feat: MultiTrajectory chi2 handling

[paulgessinger]

This PR makes MultiTrajectory always initialize the chi2 value to 0. In addition, I updated MultiTrajectoryHelpers::trajectoryState to only add track state chi2 to the overall chi2sum for measurement track states.
Outlier track states' chi2 is still recorded in the separate outlierChi2.
I think this makes sense since the outlier is not actually used in the fit, and the track state can only contain the predicted chi2 rather than the filtered chi2 that the measurement states have.

Does this make sense @XiaocongAi?

[paulgessinger]

This will change the output root file hashes, which I'll update once the CI concludes.

[codecov]

Codecov Report

Merging #1057 (9f33d21) into main (af862bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1057   +/-   ##
=======================================
  Coverage   48.57%   48.57%           
=======================================
  Files         338      338           
  Lines       17343    17343           
  Branches     8200     8200           
=======================================
  Hits         8425     8425           
  Misses       3180     3180           
  Partials     5738     5738           

Impacted Files	Coverage Δ
Core/include/Acts/EventData/MultiTrajectory.hpp	70.53% <ø> (ø)
.../include/Acts/EventData/MultiTrajectoryHelpers.hpp	50.00% <ø> (ø)

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[Corentin-Allaire]

For me this make sense but we should wait probably for @XiaocongAi comments just to be sure.