nightly: gpui: Fix `img` aspect ratio overriding existing value (#62367)
Objective The gpui::img element always overrides the aspect_ratio field, so if you have an image element that applies its own .aspect_ratio() it just gets wiped out. Solution Only apply the aspect ratio default if one is not already set. Testing It's a very minor change but I did add a small test to ensure it actually gets overridden. Self-Review Checklist: I've reviewed my own diff for quality, s…