[PWGCF] Add configurable FB eta-window width and clean warnings (#16714)

This PR adds a configurable FB pseudorapidity window width (`cfgEtaWidth`) to the strongly intensive FB correlation task. The FB windows are now defined using fixed η-center positions and a configurable width, enabling systematic studies of η-acceptance effects. Minor code cleanup and formatting updates were also applied to satisfy linter and CI requirements.

Author: isputows