Fix CCW discovery for generated bindable property values

[Tom McDonald (tommcdon)]

Description

Fixes #2458.

GeneratedBindableCustomProperty generates accessors that return bindable
property values through object, but those generated accessors cannot be
analyzed by the same source-generator pass. For getter-only collection
properties, the concrete return type was therefore omitted from
WinRTGlobalVtableLookup.g.cs unless another authored or XAML-generated
expression happened to expose it across the WinRT ABI.

This caused UWP XAML bindings such as
ItemsSource="{Binding DisplayItems}" to return an
ObservableCollection<string> without registering its IVector<string>
CCW projection. Applications had to add an unrelated property to the page
code-behind or explicitly use GeneratedWinRTExposedExternalType to root
the collection.

Changes

• Propagate the existing generated-bindable-property discovery flag through
  property initializers, expression-bodied properties, and block-bodied
  getter returns.
• Add source-generator regressions for all three property forms.
• Verify non-bindable properties do not trigger additional lookup generation.
• Verify the generated lookup includes the correct WinRT projections.

Validation

• SourceGenerator regression tests.
• Validated the repro case from Bug: Binding issue #2458 with the fix.

[Tom McDonald (tommcdon)]

Manodasan Wignarajah (@manodasanW) Sergio Pedri (@Sergio0694) Jeremy Koritzinsky (@jkoritzinsky)