TrkVertexFitterValidationUtils+TrkValEvent: Fix cppcheck 2.14 warnings.

Prefer returning const references to members rather than copies.

This merge request affects 2 packages:

• Tracking/TrkValidation/TrkValEvent
• Tracking/TrkValidation/TrkVertexFitterValidationUtils

Affected files list will not be printed in this case

added JetEtmiss PhysValidation Tracking main review-pending-level-1 labels

CI Result FAILURE (hash 7ff9816c)

	Athena
externals
cmake
make
tests

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Athena: number of compilation errors 0, warnings 0
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CI test failure due to EFTRACK-579, unrelated to this MR. Approving, L1 (Fábio).

removed review-pending-level-1 label

added review-approved label

mentioned in commit f54e95e9

merged