[PWGLF,PWGMM] Add generator-level vs reco-level flattenicity task

[PaolaVT]

No description provided.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 26 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/o2 for eaeec4a at 2026-08-05 23:03:

## sw/BUILD/O2Physics-latest/log
CMake Error at cmake/O2PhysicsAddWorkflow.cmake:34 (message):

Full log here.

[alibuild]

Error while checking build/O2Physics/code-check for eaeec4a at 2026-08-05 23:07:

## sw/BUILD/O2Physics-latest/log
CMake Error at cmake/O2PhysicsAddWorkflow.cmake:34 (message):
ninja: error: rebuilding 'build.ninja': subcommand failed

Full log here.

[vkucera]

You are supposed to check your changes before opening the PR for review.

[alibuild]

Error while checking build/O2Physics/staging for 4c0bee3 at 2026-08-06 17:48:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGMM/UE/Tasks/flattenicityTask.cxx:27:10: fatal error: Framework/Constants.h: No such file or directory
ninja: build stopped: subcommand failed.

Full log here.