24392 [ci] Remove null safety experiment flag (cla: yes)

24399 Roll buildroot to e7857d269381e23d1a89a3cbf017854992bda15a (cla: yes)

24422 Windows: Fix fml::GetFullHandlePath for WINUWP target (cla: yes)

24423 Add missing header guard, namespace (affects: desktop, affects: engine, affects: text input, cla: yes)

24424 Adjust header guards for updated paths (cla: yes)

24428 Windows: Improve window resizing (cla: yes, waiting for tree to go green)