[XSG] RelativeSource without explicit Mode parsed as invalid '(none)'

[StephaneDelcroix]

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Description

XSG fails to parse RelativeSource bindings when used with AncestorType but without an explicit Mode property. It parses the mode as '(none)' which is invalid.

Reproduction

<SwipeItem Command="{Binding Source={RelativeSource AncestorType={x:Type vm:MyViewModel}}, Path=MyCommand}" />

Error:

error MAUIG1001: An error occured while parsing Xaml: Invalid RelativeSource Mode '(none)'.

Expected Behavior

When Mode is not specified but AncestorType is, the mode should default to FindAncestor (as it does at runtime).

Affected File

src/Controls/samples/Controls.Sample/Pages/Controls/SwipeViewGalleries/SwipeViewBindingContextGallery.xaml

Workaround

Use Inflator="Runtime" for the affected XAML file, or explicitly specify Mode=FindAncestor.

Context

Discovered while testing XSG fixes for #33524.