Fix electron brems CSV files to use correct filename format

Removed .hepmc3.tree.root extension from the first column and replaced with $ to match the format used in other background files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: rahmans1

BACKGROUNDS/dataprod_rel-1.0.1_electron_brems_10x275_10000Ahr_machine_runtime_50s.csv

@@ -1 +1 @@
-BACKGROUNDS/BEAMGAS/electron/brems/EIC_ESR_Xsuite/dataprod_rel_1.0.1/10x275/10000Ahr/MachineRuntime50s/dataprod_rel_1.0.1_electron_brems_10x275_10000Ahr.hepmc3.tree.root,hepmc3.tree.root,0,0
+BACKGROUNDS/BEAMGAS/electron/brems/EIC_ESR_Xsuite/dataprod_rel_1.0.1/10x275/10000Ahr/MachineRuntime50s/dataprod_rel_1.0.1_electron_brems_10x275_10000Ahr$,hepmc3.tree.root,0,0

BACKGROUNDS/dataprod_rel-1.0.1_electron_brems_18x275_10000Ahr_machine_runtime_50s.csv

@@ -1 +1 @@
-BACKGROUNDS/BEAMGAS/electron/brems/EIC_ESR_Xsuite/dataprod_rel_1.0.1/18x275/10000Ahr/MachineRuntime50s/dataprod_rel_1.0.1_electron_brems_18x275_10000Ahr.hepmc3.tree.root,hepmc3.tree.root,0,0
+BACKGROUNDS/BEAMGAS/electron/brems/EIC_ESR_Xsuite/dataprod_rel_1.0.1/18x275/10000Ahr/MachineRuntime50s/dataprod_rel_1.0.1_electron_brems_18x275_10000Ahr$,hepmc3.tree.root,0,0

BACKGROUNDS/dataprod_rel-1.0.1_electron_brems_5x100_10000Ahr_machine_runtime_50s.csv

@@ -1 +1 @@
-BACKGROUNDS/BEAMGAS/electron/brems/EIC_ESR_Xsuite/dataprod_rel_1.0.1/5x100/10000Ahr/MachineRuntime50s/dataprod_rel_1.0.1_electron_brems_5x100_10000Ahr.hepmc3.tree.root,hepmc3.tree.root,0,0
+BACKGROUNDS/BEAMGAS/electron/brems/EIC_ESR_Xsuite/dataprod_rel_1.0.1/5x100/10000Ahr/MachineRuntime50s/dataprod_rel_1.0.1_electron_brems_5x100_10000Ahr$,hepmc3.tree.root,0,0