Fix 15116: False positive: stlFindInsert (modifying existing element) - #8975

Test results for commit f6cb7b6 (tools/test-my-pr.py, main compared to this PR):

Test: http://ec2-16-170-140-253.eu-north-1.compute.amazonaws.com/pr-8975/
packages: 100
Differing warnings: 0 (+0 -0)
Timing: -0.4% (http://ec2-16-170-140-253.eu-north-1.compute.amazonaws.com/pr-8975/2026-10-31_16-08-01-f6cb7b6c6c37_timing.html)

_{Posted automatically by the cppcheck PR test runner. +N: warnings only with this PR, -N: warnings only with main. The AI review is written by Claude and can be wrong.}