Implemented the AuthMiddleware middleware in the library target.

Tests/MarvelService/Types/Extensions/String+Samples.swift

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+extension String {
+    /// A namespace assigned for Marvel API key samples.
+    enum Key {
+        /// A Marvel API private key sample.
+        static let `private` = "SomePrivateKey"
+        /// A Marvel API public key sample.
+        static let `public` = "SomePublicKey"
+    }
+}

Tests/MarvelService/Types/Extensions/Tag+Customs.swift

@@ -19,6 +19,9 @@ extension Tag {
     /// A flag that indicates tests for a type extension.
     @Tag static var `extension`: Self
     
+    /// A flag that indicates tests for a middleware type.
+    @Tag static var middleware: Self
+    
     /// A flag that indicates tests for a use case type.
     @Tag static var useCase: Self
 }