Fix nullable typeof expressions in custom property providers

[Sergio Pedri (Sergio0694)]

Summary

Fix GeneratedCustomPropertyProvider emitting nullable reference annotations in typeof expressions for property descriptors and indexer lookup. Retain annotations in casts and preserve nullable value types.

Motivation

Valid nullable binding models currently fail to compile with CS8639 because the generator emits expressions such as typeof(string?) and typeof(object?). Runtime type lookup must omit nullable reference annotations without stripping Nullable<T> or changing init-only setter behavior.

Changes

• src\Authoring\WinRT.SourceGenerator2\CustomPropertyProviderGenerator.Execute.cs and src\Authoring\WinRT.SourceGenerator2\Models\CustomPropertyInfo.cs: carry separate Roslyn-formatted names for typeof operands alongside the existing nullable-annotated cast names.
• src\Authoring\WinRT.SourceGenerator2\CustomPropertyProviderGenerator.Emit.cs: use the runtime-type names for ordinary and indexed property descriptors and indexer parameter matching.
• src\Tests\SourceGenerator2Test\Test_CustomPropertyProviderGenerator.cs: add 18 compilation-based and runtime regression cases covering nullable strings, objects, arrays, generics, structs, and tuples under automatic discovery and explicit selection. Assert descriptor type identity, null round-trips, getter/setter behavior, and read-only init properties.

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