Fix SwiftUI Previews

[xquezme]

Summary

Replaced internal main-thread assertions using ASDisplayNodeCAssertMainThread() with explicit [NSThread isMainThread] checks in ASInternalHelpers.mm.

Why this change

SwiftUI Previews run in a separate lightweight process (PreviewHost / XCPreviewAgent). While Thread.isMainThread / [NSThread isMainThread] returns true (because PreviewHost fakes the Foundation main thread for compatibility with SwiftUI, UIKit, and @MainActor), lower-level or custom main-thread checks (like the previous macro) could fail or behave inconsistently.

This caused assertion failures or skipped initialization/cleanup logic during preview rendering, leading to crashes or broken previews.

pthread_main_np()	[NSThread isMainThread]
Returns false (0)	Returns true
Checks if current pthread is the original process bootstrap thread	Checks if current thread is the Foundation-designated main thread
Not affected by PreviewHost fake-main-thread logic	Fully affected — PreviewHost overrides/fakes it as main thread
Does not match SwiftUI / MainActor expectations	Matches SwiftUI / UIKit / MainActor expectations