[Bug] Model fitting is broken with scipy 1.15

[Balandat]

🐛 Bug

Model fitting fails in scipy 1.15 (see test failure here: see e.g. https://github.com/pytorch/botorch/actions/runs/12601444508), presumably b/c L-BFGS-B was ported to C: https://scipy.github.io/devdocs/release/1.15.0-notes.html#scipy-optimize-improvements

#2665 pins scipy to <1.15 to avoid test failures in CI for the time being, but we need to root cause and fix it in a way that's compatible with both <1.15 and 1.15+.