Treat init-only custom properties as read-only

[Sergio Pedri (Sergio0694)]

Summary

Keep public init-only properties and indexers readable through generated ICustomProperty implementations without reporting them as writable or emitting invalid assignments.

Motivation

The custom property provider generator currently classifies any public setter as writable, including init accessors. This produces CS8852 errors for immutable classes and readonly structs, including required init-only properties, and blocks otherwise valid XAML bindings. These members must remain discoverable and readable, with writes rejected just like getter-only members.

Changes

• src\Authoring\WinRT.SourceGenerator2\CustomPropertyProviderGenerator.Execute.cs: require a public, non-init setter for CanWrite, reusing the existing read-only emission for both properties and indexers.
• src\Tests\SourceGenerator2Test\Helpers\CSharpGeneratorTest{TGenerator}.cs: add an opt-in helper that checks compilation/emit diagnostics and loads the resulting assembly for behavior assertions, leaving existing snapshot validation unchanged.
• src\Tests\SourceGenerator2Test\Test_CustomPropertyProviderGenerator.cs: add nine regression cases covering classes, readonly structs, required members, inherited properties, explicit selection, nonpublic setters, and init-only indexers. Assert that initialized values remain readable, init-only writes throw NotSupportedException, and ordinary property/indexer setters remain writable.

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