[PWGJE] Updated GNN b-jet histograms: Resolved issues for merging TH3 histograms on Hyperloop

[choich08365]

PWGJE/Tasks/bjetTaggingGnn.cxx

• Reverted TH3 histograms to multiple TH2 histograms as same as before the last update, to resolve bugs occurring when merging TH3 histograms on Hyperloop.
• Corrected axis titles.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 6 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 0bf6faa at 2026-08-25 15:02:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 0 warnings.'
Found 1 errors and 0 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/111186a293dc537db9f3cd65fef2aa935dddaadc/O2Physics-code-check/errors.txt
PWGJE/Tasks/bjetTaggingGnn.cxx:46:1: error: included header TH3.h is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.

[nzardosh]

Dear @choich08365 after this PR can you make another one to address the code-check errors please