fix(fabric): borders and shadows render consistently

[Saadnajmi]

Summary:

Resolves #2738

Borders and box shadows didn't render consistently. shadows would disappear if i resized the view, and most borders didn't render at all. I asked VSCode Copilot with Claude Opus 4.5 to debug, and after lots of back and forth, we are here..

Summary
This PR fixes rendering issues with borders and box shadows on macOS in Fabric. The core problem was that NSImage's CGImageForProposedRect: returns a new autoreleased CGImageRef each time it's called, which causes issues when used with CALayer.contents - the CGImage would get deallocated while still being referenced by Core Animation.

Changes

1. RCTUIImage - NSImage subclass with cached CGImage (RCTUIKit.h, RCTUIKit.m)
   Created RCTUIImage, a subclass of NSImage that caches its CGImageRef representation
   The cached CGImage is retained and only released on dealloc, ensuring it stays valid while used by CALayer.contents
   Added .CGImage property that returns the cached CGImage
   Updated UIImageGetCGImageRef() to use the new .CGImage property
   Updated UIImageWithContentsOfFile() and UIImageWithData() to return RCTUIImage
   Changed @compatibility_alias UIImage from NSImage to RCTUIImage
2. RCTUIGraphicsImageRenderer improvements (RCTUIKit.m)
   Changed imageWithActions: to return RCTUIImage instead of NSImage
   Replaced the deferred drawingHandler: block approach with immediate bitmap context rendering
   This ensures the image data is fully rendered and available for CGImage extraction
   Properly handles scale factor and coordinate system flipping
3. RCTViewComponentView fixes (RCTViewComponentView.mm)
   Border layer frame: Added explicit frame update for _borderLayer on macOS to handle view resizing
   Box shadow z-position: Fixed _boxShadowLayer.zPosition to be behind content (BACKGROUND_COLOR_ZPOSITION - 1)
   Container view clipping: Fixed clipsToBounds and layer.masksToBounds handling for macOS:
   When _useCustomContainerView is true (boxShadow + overflow:hidden), the container view handles clipping while the main layer stays unclipped for the shadow
   Explicitly set wantsLayer = YES on container view
   CGImage usage: Updated to use image.CGImage directly instead of UIImageGetCGImageRef(image)
   Root Cause
   On macOS, NSImage's CGImageForProposedRect:context:hints: creates a new autoreleased CGImageRef each call. When this was assigned to CALayer.contents, the CGImage would be deallocated during the next autorelease pool drain, leaving the layer with a dangling pointer. This caused rendering artifacts and potential crashes.

Testing
Tested borders with various styles (solid, dashed, different colors per side)
Tested box shadows (inset and outset) with overflow:hidden
Tested combined borders + shadows + clipping scenarios

Test Plan:

RNTester seems better :)

Screen.Recording.2025-11-26.at.3.39.04.PM.mov