[PWGLF] FIx he3 momentum in the KF fitting

[fmazzasc]

@creetz16

[github-actions]

O2 linter results: ❌ 2 errors, ⚠️ 6 warnings, 🔕 2 disabled

[alibuild]

Error while checking build/O2Physics/code-check for db19e74 at 2026-08-14 18:25:

++ set -euo pipefail
++ [[ -d /sw/slc9_x86-64/Clang/v20.1.7-24 ]]
+++ realpath /sw/BUILD/052508d95fbcb122f42e2ace28cf8bd6cc254496/O2Physics-code-check/../../O2Physics-latest/O2Physics
++ O2PHYSICS_BUILDDIR=/sw/BUILD/d559ba52e4c6e250b3e5b46dfc4b712292bc055f/O2Physics
++ cp /sw/BUILD/d559ba52e4c6e250b3e5b46dfc4b712292bc055f/O2Physics/compile_commands.json .
+++ python3 -c 'import json, os; print(os.path.commonpath([x["file"] for x in json.loads(open("compile_commands.json").read()) if "sw/BUILD" not in x["file"] and "G__" not in x["file"] and x["file"].endswith(".cxx")]))'
++ O2PHYSICS_SRC=/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0
++ pushd /sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0
/sw/SPECS/slc9_x86-64/O2Physics-code-check/1.0-local639/O2Physics-code-check.sh: line 16: pushd: /sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0: No such file or directory

Full log here.