Add tests for collection expressions targeting read-only collections

[Sergio0694]

Summary

Add CoreCLR unit tests and trim/AOT functional tests covering collection expressions whose target type is a read-only collection interface (IEnumerable<T>, IReadOnlyList<T>, IReadOnlyCollection<T>), and validate that they marshal correctly across the WinRT ABI.

Motivation

Collection expressions targeting a read-only collection interface are lowered by the C# compiler to internal synthesized backing types: <>z__ReadOnlyArray<T> for multiple elements and <>z__ReadOnlySingleElementList<T> for a single element. When such an object crosses the WinRT ABI it needs a COM Callable Wrapper (CCW).

In CsWinRT 2.x the source generator could not see these compiler-synthesized types (they are produced during lowering, after generators run), so marshalling them emitted AOT warnings and fell back to runtime vtable generation, which is not Native AOT safe. In CsWinRT 3.0 the interop generator discovers them directly from IL (for example, the newobj in the lowered collection expression) and generates their CCWs ahead of time, so they are fully trim and AOT safe.

There was no existing test coverage for this scenario anywhere in the suite (existing collection-expression usages only ever target concrete types such as List<T> or arrays). This PR adds regression coverage for both normal CoreCLR execution and trimmed/Native AOT publishing. Because both synthesized types implement IEnumerable<T>, IReadOnlyList<T>, and IList<T>, the tests also assert that the CCW exposes IIterable<T>, IVectorView<T>, and IVector<T> (not just IIterable<T>), for both the multi-element and single-element backing types.

Changes

• src/Tests/UnitTest/TestComponentCSharp_Tests.cs: new TestCollectionExpressionReadOnlyInterfaceMarshalling test. Marshals collection expressions typed as each read-only interface (multi-element and single-element) through Class.GetIteratorForCollection (native IIterable<T> consumption) and BindableIterableProperty (bindable round-trip), and uses ConvertToUnmanaged plus Marshal.QueryInterface to assert the synthesized type's CCW exposes IIterable<int>, IVectorView<int>, and IVector<int>.
• src/Tests/UnitTest/TestComponent_Tests.cs: new Collections_ReadOnly_List_Call_CollectionExpression test. Returns IReadOnlyList<string> collection expressions from Collection6Call so the synthesized backing type is marshalled back to native as a CCW exposing IVectorView<string>.
• src/Tests/FunctionalTests/Collections/Program.cs: mirrors the above scenarios so the interop generator's CCW discovery for the synthesized backing types is exercised under trimming and Native AOT, including a QueryInterface-based check that the synthesized type's CCW exposes IIterable<int>, IVectorView<int>, and IVector<int>.