fix(#3143): ensure open.no_window_picker respects window_picker.exclude

[LucasMABF]

Description

This PR ensures that when using open.no_window_picker, the fallback window selection respects the window_picker.exclude configuration.
Previously, if no window picker was used, nvim-tree would open the file in the last active window, even if that window matched an excluded type (e.g., a terminal).
Now it sets the fallback window as the first available usable window, not excluded in the configuration, which can prevent undesired behavior.

Related Issue

Fixes #3143

Additional Notes

I removed the first_win_id function to avoid redundant calls to usable_win_ids. Happy to adjust if you'd like to keep it.

Let me know if there’s any other adjustment or further cleanup you’d like.
Appreciate your time and review!