Remove duplicate accessibility declarations on macOS; NativeViewAccessible becomes a smart pointer holding id<NSAccessibility>

The impact of this change on WebView2Mac is mainly reflected in changes to type compatibility and memory management methods. Since downstream WebView2Mac code extensively inherits and calls remote_cocoa’s NativeViewAccessible-related interfaces, all operations involving accessibility bridging, passing, comparison, assignment, etc., need to be adapted to the OwnedNSAccessibility smart pointer semantics (such as .Get(), construction, null checks, etc.). If downstream code still assumes NativeViewAccessible is a raw pointer, it may result in compilation errors or inconsistent behavior. In addition, the introduction of smart pointers improves memory safety and reduces the risk of dangling pointers and double-free issues. In terms of functionality and performance, there is theoretically no direct negative impact, but it is necessary to ensure all downstream adaptations are in place; otherwise, compilation or runtime issues may occur. It is recommended to focus regression testing on accessibility-related features, especially the lifecycle management of windows, views, and NSAccessibility objects.