Fix unwanted behaviour in signal filtering with embedding pattern different from @0:e1

[fgrosa]

In #14698 I wrongly thought that I could exclude the particles from background event to count the collisions to reduce the size of the vector, without considering that if the embedding pattern different from @0:e1, there are collisions with only background particles.
This PR fixes this bug @sawenzel

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for f7f0765 at 2025-10-14 23:16:

++ cp /sw/slc9_x86-64/O2/14735-slc9_x86-64-local1/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")]))'
++ O2_SRC=/sw/SOURCES/O2/14735-slc9_x86-64/0
++ [[ -e /sw/SOURCES/O2/14735-slc9_x86-64/0/CMakeLists.txt ]]

Full log here.