Fix missing Keys/Values on projected dictionary types

[Sergio0694]

Summary

Fixes a latent bug in projected dictionary types where the first access to Keys or Values on a runtime class projected as IDictionary<K, V> or IReadOnlyDictionary<K, V> (e.g., Windows.Foundation.Collections.PropertySet, ValueSet, StringMap) would throw MissingMethodException because the UnsafeAccessor stub emitted by cswinrt.exe referenced a static method that the interop generator never actually emitted.

Motivation

cswinrt.exe's write_*_using_static_abi_methods paths declare [UnsafeAccessor(...)] static extern ... Keys/Values(...) stubs that target ABI.System.Collections.Generic.<#corlib>IDictionary'2<K|V>Methods.Keys (and Values) in the interop sidecar (WinRT.Interop.dll). The interop generator's InteropTypeDefinitionBuilder.IDictionary2.Methods and InteropTypeDefinitionBuilder.IReadOnlyDictionary2.Methods only emit Item, Count, ContainsKey, TryGetValue (plus mutation surface for the mutable variant) — Keys and Values were missing. The UnsafeAccessor binder lazy-resolves on first call, so this only fails at runtime, on the first Keys or Values access.

While investigating, the IReadOnlyDictionary UnsafeAccessor stubs were also discovered to declare the wrong return type (ICollection<T>, copy-pasted from the IDictionary template). IReadOnlyDictionary<TKey, TValue>.Keys/.Values return IEnumerable<T>, and ReadOnlyDictionaryKeyCollection<TKey, TValue>/ReadOnlyDictionaryValueCollection<TKey, TValue> only implement IEnumerable<T>. UnsafeAccessor is signature-sensitive, so even if the interop generator emitted these methods with the correct IEnumerable<T> return type, the existing stub would never bind.

Changes

Commits are ordered so the test commit precedes the fixes — CI on this PR (with tests-only applied) should reproduce the failure before the fix commits land.

• src/Tests/: new unit tests in TestComponentCSharp_Tests.cs covering the IDictionary path via StringMap / PropertySet / ValueSet (enumeration, count, contains, read-only contract, mutation reflection, repeated access, empty case, explicit cast); new C++ runtime class TestComponentCSharp.CustomReadOnlyDictionaryTest implementing IMapView<String, String> to provide direct coverage for the IReadOnlyDictionary path; new unit tests using it (direct, explicit cast, negative-type assertion pinning the IEnumerable<T> (not ICollection<T>) contract, empty case); functional/AOT tests in FunctionalTests/Collections/Program.cs mirroring both paths.
• src/cswinrt/code_writers.h: fix write_readonlydictionary_members_using_static_abi_methods to declare the Keys/Values UnsafeAccessor stubs with return type IEnumerable<T> (matching the property body and the runtime collection types). The IDictionary path is unchanged because its ICollection<T> return type is already correct.
• src/WinRT.Interop.Generator/Builders/InteropTypeDefinitionBuilder.IDictionary2.cs and InteropTypeDefinitionBuilder.IReadOnlyDictionary2.cs: emit the missing Keys and Values static methods on the generated Methods types. Each method constructs the appropriate runtime collection wrapper around a fresh NativeObject built from the supplied WindowsRuntimeObjectReference (e.g., new DictionaryKeyCollection<TKey, TValue>(new NativeObject(thisReference))). The NativeObject inherits from WindowsRuntimeDictionary<...> / WindowsRuntimeReadOnlyDictionary<...>, which implement the interface required by the collection constructors.
• src/WinRT.Interop.Generator/Generation/InteropGenerator.Emit.cs: reorder the emission so NativeObject runs before Methods for both IDictionary2 and IReadOnlyDictionary2 (the new Keys/Values method bodies depend on nativeObjectType). The NativeObject builder does not depend on the Methods type, so this reordering is safe.