[dev-v5] FluentDataGrid Cyclomatic Complexity #4700

It would be good to adapt and simplify these methods to ensure accurate Crap Score and Cyclomatic Complexity values. Copilot could probably help with this task.

Simplifying these methods will make the code easier to read, test, and maintain, while reducing the risk of introducing bugs when we need to make changes — which is exactly what lower Cyclomatic Complexity and CRAP scores reflect.

• Cyclomatic Complexity — Microsoft's own documentation

• Code Metrics overview (including CRAP Score context)