fix(ios): prewarm modal blur snapshots

[futamura]

Summary

• Prewarm blurred background snapshots before modal visibility becomes nonzero.
• Add regression coverage for hidden blurred backgrounds preparing a snapshot.

Verification

• git diff --check
• xcodebuild test -project Fluidable.xcodeproj -scheme Fluidable -destination platform=iOS Simulator,id=5F3F8F67-5FEC-459B-AF9B-E7E7D4BCEBBF -only-testing:FluidableTests/UIKitSpec CODE_SIGNING_ALLOWED=NO
• xcodebuild test-without-building via xctestrun shard 0/20 for FluidableUITests/MainSpec, NavigationFluidModal portrait, 3 tests

UI check

• NavigationFluidModal FinishAnimatedPresent_FinishAnimatedDismissWithBackground: OK
• NavigationFluidModal FinishAnimatedPresent_FinishInteractiveDismiss: OK
• NavigationFluidModal KeepRootNavigationVisibleAndAlignFluidModalContent: OK