fix: prevent MeasureOverride from returning Infinity to Avalonia layout (#2136)

[gadfly3173]

When VirtualizingStackPanel.ScrollIntoView() measures items, it passes Size.Infinity as the available size. Several custom controls were passing availableSize.Width (or the entire availableSize) straight back in their MeasureOverride return value, which violates Avalonia's layout contract and throws InvalidOperationException.

This was most visible on the stash page: selecting stash the 13th+ (beyond the viewport, default window size would be 6+), switching away, then switching back would crash the app because AutoScrollToSelectedItemIfNecessary triggered ScrollIntoView on the virtualized item.

P.S.:
Also applied defensive fixes to CommitRefsPresenter and ImageContainer, which are not related to the reported crash but have the same contract violation — MeasureOverride may return Infinity under certain conditions, you can remove them if you want :

• CommitRefsPresenter: when AllowWrap is true, the wrapped branch returns availableSize.Width directly. In practice this branch is unreachable with Infinity input (wrapping never triggers when width is unbounded), but the code still references availableSize.Width in a return path, which is a latent contract violation.

• ImageContainer: return availableSize as-is when image properties are null. These controls are currently only used in diff views (not in virtualized lists), so the issue is unlikely to surface, but returning raw availableSize from MeasureOverride is always wrong per Avalonia's layout contract.