Explicitly require Transferable values for attachments also be Sendable (#1688)

Author: stmontgomery

Sources/Overlays/_Testing_CoreTransferable/Attachments/Attachment+Transferable.swift

@@ -61,7 +61,7 @@ extension Attachment {
     as contentType: UTType? = nil,
     named preferredName: String? = nil,
     sourceLocation: SourceLocation = #_sourceLocation
-  ) async throws where T: Transferable, AttachableValue == _AttachableTransferableWrapper<T> {
+  ) async throws where T: Transferable & Sendable, AttachableValue == _AttachableTransferableWrapper<T> {
     let transferableWrapper = try await _AttachableTransferableWrapper(exporting: transferableValue, as: contentType)
     self.init(transferableWrapper, named: preferredName, sourceLocation: sourceLocation)
   }

Sources/Overlays/_Testing_CoreTransferable/Attachments/_AttachableTransferableWrapper.swift

@@ -26,7 +26,7 @@ import UniformTypeIdentifiers
 ///   @Available(Swift, introduced: 6.4)
 /// }
 @available(_transferableAPI, *)
-public struct _AttachableTransferableWrapper<T>: Sendable where T: Transferable {
+public struct _AttachableTransferableWrapper<T>: Sendable where T: Transferable & Sendable {
   /// The transferable value.
   private var _transferableValue: T