[tmva][sofie] fix: correct Mod/FMod Func() constant-folding, add regression test

[harz05]

Changes:

In tmva/sofie/inc/TMVA/ROperator_BasicBinary.hxx, the Func() method for the Mod and FMod traits was calling std::pow(t1, t2) instead of the correct modulo operation. Func() is used during constant-folding when both inputs to a Mod/FMod node are ONNX Constant nodes, producing completely wrong output silently.

• Mod::Func fixed to return t1 % t2
• FMod::Func fixed to return std::fmod(t1, t2);

I've also included a regression test FMod_ConstantFolding that exercises exactly this code path.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #21734