Implemented the CheckURIUseCase use case type in the library target.

Sources/DocCMiddleware/Internal/Extensions/String+Constants.swift

@@ -0,0 +1,20 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+extension String {
+    /// An empty string.
+    static let empty = ""
+    /// A forwarding slash.
+    static let forwardSlash = "/"
+    /// An indication of a previous folder in a path component.
+    static let previousFolder = ".."
+}

Sources/DocCMiddleware/Internal/Use Cases/CheckURIUseCase.swift

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+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+import struct HummingbirdCore.URI
+
+/// A use case that checks whether a given URI against a set of conditions, to determine whether the URI could be used by the middleware or not.
+struct CheckURIUseCase {
+
+    // MARK: Functions
+    
+    /// Checks whether a provided URI against a set of conditions, so it could be used by the middleware.
+    /// - Parameter uri: A URI to check.
+    /// - Returns: A non-encoded URI, which is ready to be used by the middleware.
+    func callAsFunction(_ uri: URI) -> String? {
+        guard
+            let uriPath = uri.path.removingPercentEncoding,
+            !uriPath.contains(.previousFolder),
+            uriPath.hasPrefix(.forwardSlash)
+        else {
+            return nil
+        }
+        
+        return uriPath
+    }
+    
+}