Fix flaky test_benchmark_gradient / Weber_BMC2015

[dweindl]

Frequent failures specifically with Python3.14. Change RNG seed. Closes #3078.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.71%. Comparing base (6dd5b87) to head (1969340).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3137      +/-   ##
==========================================
- Coverage   77.38%   76.71%   -0.67%     
==========================================
  Files         318      318              
  Lines       20631    20631              
  Branches     1483     1483              
==========================================
- Hits        15965    15827     -138     
- Misses       4658     4796     +138     
  Partials        8        8              

Flag	Coverage Δ
cpp	70.44% <ø> (-0.03%)	⬇️
cpp_python	37.80% <ø> (ø)
petab	47.16% <ø> (ø)
python	68.90% <ø> (-0.03%)	⬇️
sbmlsuite-jax	?

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see 6 files with indirect coverage changes

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