fix compilation on 6.1 nightly and 6.2

Author: sebsto

Sources/OpenAPILambdaTransport.swift

@@ -54,19 +54,25 @@ public struct OpenAPILambdaTransport: ServerTransport, Sendable {
         // call the actor-isolated `router.add()` function from a synchronous context
         // because ServerTransport.resgister() is synchronous
         let semaphore = DispatchSemaphore(value: 0)
-        var thrownError: Error? = nil
-        Task {
+        let errorBox = Box<Error?>(nil)
+
+        Task { @Sendable in
             do {
                 try await self.router.add(method: method, path: path, handler: handler)
             }
             catch {
-                thrownError = error
+                errorBox.value = error
             }
             semaphore.signal()
         }
         semaphore.wait()
-        if let error = thrownError {
+        if let error = errorBox.value {
             throw error
         }
     }
+
+    final class Box<T>: @unchecked Sendable {
+        var value: T
+        init(_ value: T) { self.value = value }
+    }
 }