Set the EMCAL sampling factor for the G4HepEm physics list

This adds the Geant4 sampling factor for FTFP_BERT_EMV_HEP+optical.

- The value 0.810 gives the same EMCAL energy per primary as FTFP_BERT_EMV+optical with 0.821.
- It was determined with e- of 5 and 20 GeV and gamma of 5 GeV.
- Before, the list fell back to the default by name, without a check.

https://its.cern.ch/jira/browse/O2-7210
https://its.cern.ch/jira/browse/O2-7316

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/EMCAL/base/src/Geometry.cxx

@@ -290,6 +290,9 @@ void Geometry::DefineSamplingFraction(const std::string_view mcname, const std::
       samplingFactorTranportModel = 0.821;
     } else if (physicslist == "FTFP_BERT_EMV+optical+biasing" || physicslist == "FTFP_INCLXX_EMV+optical") {
       samplingFactorTranportModel = 0.81;
+    } else if (physicslist == "FTFP_BERT_EMV_HEP+optical") {
+      // e-, e+ and gamma with G4HepEm; matches FTFP_BERT_EMV+optical with 0.821 (O2-7210)
+      samplingFactorTranportModel = 0.810;
     }
   }