Implemented the User Agent middleware (#6)

This PR contains the work done to implement the `UserAgentMiddleware` middleware that includes user agent information into a header of the requests sent by the `Client` type, as defined in the [Discogs documentation](https://www.discogs.com/developers/#page:home,header:home-general-information). For this purpose, the `CamelCaseValidationRule`, `SemanticVersionValidationRule` and `URLValidationRule` types were implemented and integrated into the existing `ValidateInputUseCase` type.

Reviewed-on: #6
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/DiscogsService/Public/Errors/InputValidationError.swift

@@ -0,0 +1,33 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+/// A representation of all the possible validation error that could be thrown while validating an input.
+public enum InputValidationError: Error {
+    /// An input is empty.
+    case inputIsEmpty
+    /// An input is nil.
+    case inputIsNil
+    /// An input is not camel-case.
+    case inputNotCamelCase
+    /// An input does not comply with the consumer key requirements.
+    case inputNotConsumerKey
+    /// An input does not comply with the consumer secret requirements.
+    case inputNotConsumerSecret
+    /// An input is not a semantic version.
+    case inputNotSemanticVersion
+    /// An input is not a URL.
+    case inputNotURL
+    /// An input does not comply with the user token requirements.
+    case inputNotUserToken
+}

Sources/DiscogsService/Public/Middlewares/AuthMiddleware.swift

@@ -43,6 +43,7 @@ public struct AuthMiddleware {
     /// - Parameters:
     ///   - method: A representation of an authentication method to use to authenticate requests.
     ///   - transport: A representation of a transport option to send credentials in requests.
+    /// - Throws: An error of type ``InputValidationError`` in case an input failed any validation.
     public init(
         method: AuthMethod = .none,
         transport: AuthTransport

Sources/DiscogsService/Public/Middlewares/UserAgentMiddleware.swift

@@ -0,0 +1,109 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+import class OpenAPIRuntime.HTTPBody
+
+import protocol OpenAPIRuntime.ClientMiddleware
+
+import struct Foundation.URL
+import struct HTTPTypes.HTTPField
+import struct HTTPTypes.HTTPFields
+import struct HTTPTypes.HTTPRequest
+import struct HTTPTypes.HTTPResponse
+
+/// A middleware that attaches the user agent header into the requests to the service.
+///
+/// Please refer to the [Discogs documentation](https://www.discogs.com/developers/#page:home,header:home-general-information) for further information.
+public struct UserAgentMiddleware {
+    
+    // MARK: Properties
+
+    /// A formatted value for the user agent header.
+    let agentField: HTTPField
+    
+    // MARK: Initializers
+    
+    /// Initializes this middleware.
+    /// - Parameter product: A product from which the user agent will be generated from.
+    /// - Throws: An error of type ``InputValidationError`` in case an input failed any validation.
+    public init(product: Product) throws {
+        let agentName = ValidateInputUseCase(rules: .notNil, .notEmpty, .camelCase)
+        let agentVersion = ValidateInputUseCase(rules: .notNil, .notEmpty, .semanticVersion)
+        let agentURL = ValidateInputUseCase(rules: .notNil, .notEmpty, .url)
+        
+        try agentName(product.name)
+        try agentVersion(product.version)
+        try agentURL(product.url)
+        
+        self.agentField = .init(
+            name: .userAgent,
+            value: .init(format: .Format.userAgent, product.name, product.version, product.url)
+        )
+    }
+    
+}
+
+// MARK: - ClientMiddleware
+
+extension UserAgentMiddleware: ClientMiddleware {
+    
+    // MARK: Functions
+    
+    public func intercept(
+        _ request: HTTPRequest,
+        body: HTTPBody?,
+        baseURL: URL,
+        operationID: String,
+        next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
+    ) async throws -> (HTTPResponse, HTTPBody?) {
+        return try await next(
+            .init(
+                method: request.method,
+                scheme: request.scheme,
+                authority: request.authority,
+                path: request.path,
+                headerFields: userAgentHeader(request.headerFields)
+            ),
+            body,
+            baseURL
+        )
+    }
+    
+}
+
+// MARK: - Helpers
+
+private extension UserAgentMiddleware {
+    
+    // MARK: Functions
+    
+    /// Adds a user agent header to the existing header fields.
+    /// - Parameter fields: A set of header fields to update.
+    /// - Returns: An updated set of header fields including the user agent header.
+    func userAgentHeader(_ fields: HTTPFields) -> HTTPFields {
+        var fields = fields
+        
+        fields.append(agentField)
+
+        return fields
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension String.Format {
+    /// A format for the user agent header.
+    static let userAgent = "%@/%@ +%@"
+}

Sources/DiscogsService/Public/Models/Product.swift

@@ -33,8 +33,8 @@ public struct Product: Sendable {
     /// Initializes this model.
     /// - Parameters:
     ///   - name: A camel-cased name of a product.
-    ///   - version: A URI link related to a product.
-    ///   - url: A semantic version of a product.
+    ///   - version: A semantic version of a product.
+    ///   - url: A URI link related to a product.
     public init(
         name: String,
         version: String,