[PWGLF] Implement validation histograms and code cleanup

[PaolaVT]

This update introduces new histograms to monitor which events and tracks pass the selection criteria. The histograms are filled after event-level and track-level cuts and are meant for validation and debugging.
Additionally, code improvements were made by replacing hard-coded bin indices with enums and using std::hypot for better readability and precision.

[github-actions]

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

[vkucera]

@PaolaVT Which issue are you referring to in the PR title?

[PaolaVT]

@vkucera The macOS and macOS-arm checks failed due to an undeclared variable in the code

[vkucera]

@vkucera The macOS and macOS-arm checks failed due to an undeclared variable in the code

That failure does not seem to be related to the actual purpose of this PR.
Please change the PR title according to the contribution instructions.

[PaolaVT]

@vkucera The macOS and macOS-arm checks failed due to an undeclared variable in the code

That failure does not seem to be related to the actual purpose of this PR. Please change the PR title according to the contribution instructions.

Thanks a lot for the suggestions, I have implemented them to improve the analysis accordingly.