[Windows] Fix AspectFit image fills container regression causing UI test failures on candidate branch

[praveenkumarkarunanithi]

Note

Are you waiting for the changes in this PR to be merged?
It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue.
Thank you!

Root Cause

PR #34699 introduced _cachedImageSize to eliminate timing-dependent behavior in GetDesiredSize(). As part of that change, MapSourceAsync() resets the cache to Zero whenever a new image source is assigned.

However, GetDesiredSize() was updated to rely exclusively on _cachedImageSize and no longer considered the live decoded size available on BitmapSource. This created a timing gap where WinUI had already completed image decoding and populated BitmapSource.PixelWidth/PixelHeight, but the asynchronous ImageOpened event had not yet fired to update the cache.

If a layout pass occurred during this window, GetDesiredSize() observed a zero-sized cache, skipped the natural-size constraint, and allowed the image to expand to the available container size instead of respecting its decoded pixel dimensions.

Description of Change

Two minimal updates were made in ImageHandler.Windows.cs:

GetDesiredSize()

• Reads the live decoded size from BitmapSource first.
• Falls back to _cachedImageSize only when the live size is not yet available.
• Aligns with the existing live-first, cache-fallback pattern already used in UpdatePlatformMaxConstraints().

GetImageSize()

• Added a null-conditional guard for PlatformView to prevent a potential null-reference exception when GetDesiredSize() is called after handler disconnection.

Regression Introduced By

PR #34699

Issues Fixed

Fixes the following candidate branch regressions:
In Issue30403SmallImages.cs file:
ConstrainedContainers_ShouldHandleSmallImages
SmallImageEndAlignment_ShouldRespectAlignment
SmallImageInLargeContainer_ShouldNotExceedIntrinsicSize
SmallImageStartAlignment_ShouldRespectAlignment
SizeComparison_ShouldShowDifferentSizes
TinyImage_ShouldRemainTiny

Tested the behaviour in the following platforms

• Android
• Windows
• iOS
• Mac