[PWGCF] Add Nch-dependent rebinning of radial-flow correction maps

[somabhatta]

For radialFlowDecorr.cxx implement: Rebin efficiency and fake-rate maps using inverse-variance weights, Exclude zero-error inputs, reject invalid correction bins, and enable histogram error storage.

[github-actions]

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

[alibuild]

Error while checking build/O2Physics/code-check for 05540b4 at 2026-09-17 04:37:

## 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/2738761f55e3e43efe8b8a870df8a5c3ca8762af/O2Physics-code-check/errors.txt
PWGCF/EbyEFluctuations/Tasks/radialFlowDecorr.cxx:1118:26: error: no header providing "kTRUE" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.

[vkucera]

@somabhatta Please fix the errors before merging.