[GUI] Implment Slider::respect_grabber_area_min_size

[Delsin-Yu]

Implement

• [GUI] Add an option to make the slider respect grabber area's min size godot-proposals#13484

Adds an option to the Slider that, when enabled, will respect the min-size reported by the stylebox when rendering the grabber area.

QQ20251027-194832.mp4

• Bugsquad edit, closes: [GUI] Add an option to make the slider respect grabber area's min size godot-proposals#13484

[Delsin-Yu]

Context: No Handle is implmented by assigning a one-pixel-transparent texture to the handle.