Refactor WinRT metadata attributes and conditionals

Adjust conditional compilation and attribute placement for WinRT metadata types. Add conditional import of System.Runtime.Versioning and split ApiContractAttribute between implementation (WindowsRuntimeMetadata) and reference (SupportedOSPlatform + ContractVersion) builds, moving AttributeUsage to a shared location. Do the same reordering for ContractVersionAttribute, and remove redundant remarks comments from FoundationContract and UniversalApiContract to clean up source.

Author: Sergio0694

src/WinRT.Runtime2/Windows.Foundation/FoundationContract.cs

@@ -10,9 +10,6 @@ namespace Windows.Foundation;
 /// <summary>
 /// Represents the Windows Foundation API contract.
 /// </summary>
-/// <remarks>
-/// This type is required for ABI projection of the value types and delegates, but marshalling it is not supported.
-/// </remarks>
 #if WINDOWS_RUNTIME_REFERENCE_ASSEMBLY
 [ApiContract]
 [ContractVersion(262144u)]

src/WinRT.Runtime2/Windows.Foundation/Metadata/ApiContractAttribute.cs

@@ -2,16 +2,21 @@
 // Licensed under the MIT License.
 
 using System;
+#if WINDOWS_RUNTIME_REFERENCE_ASSEMBLY
 using System.Runtime.Versioning;
+#endif
 using WindowsRuntime;
 
 namespace Windows.Foundation.Metadata;
 
 /// <summary>
 /// Specifies that the type represents an API contract.
 /// </summary>
+#if WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY
 [WindowsRuntimeMetadata("Windows.Foundation.FoundationContract")]
-[AttributeUsage(AttributeTargets.Enum, AllowMultiple = false)]
+#elif WINDOWS_RUNTIME_REFERENCE_ASSEMBLY
 [SupportedOSPlatform("Windows10.0.10240.0")]
 [ContractVersion(typeof(FoundationContract), 65536u)]
+#endif
+[AttributeUsage(AttributeTargets.Enum, AllowMultiple = false)]
 public sealed class ApiContractAttribute : Attribute;

src/WinRT.Runtime2/Windows.Foundation/Metadata/ContractVersionAttribute.cs

@@ -16,12 +16,11 @@ namespace Windows.Foundation.Metadata;
 /// </summary>
 #if WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY
 [WindowsRuntimeMetadata("Windows.Foundation.FoundationContract")]
-#endif
-[AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
-#if WINDOWS_RUNTIME_REFERENCE_ASSEMBLY
+#elif WINDOWS_RUNTIME_REFERENCE_ASSEMBLY
 [SupportedOSPlatform("Windows10.0.10240.0")]
 [ContractVersion(typeof(FoundationContract), 65536u)]
 #endif
+[AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
 public sealed class ContractVersionAttribute : Attribute
 {
     /// <summary>

src/WinRT.Runtime2/Windows.Foundation/UniversalApiContract.cs

@@ -10,9 +10,6 @@ namespace Windows.Foundation;
 /// <summary>
 /// Represents the Universal API contract.
 /// </summary>
-/// <remarks>
-/// This type is required for ABI projection of the value types and delegates, but marshalling it is not supported.
-/// </remarks>
 #if WINDOWS_RUNTIME_REFERENCE_ASSEMBLY
 [ApiContract]
 [ContractVersion(1245184u)]