Implemented the AgentNameValidationRule type in the library target.

Sources/DiscogsService/Internal/Errors/InputValidationError.swift

@@ -18,6 +18,8 @@ enum InputValidationError: Error {
     case inputIsEmpty
     /// An input is nil.
     case inputIsNil
+    /// An input does not comply with the user agent name requirements.
+    case inputNotAgentName
     /// An input does not comply with the consumer key requirements.
     case inputNotConsumerKey
     /// An input does not comply with the consumer secret requirements.

Sources/DiscogsService/Internal/Validation Rules/AgentNameValidationRule.swift

@@ -0,0 +1,76 @@
+// ===----------------------------------------------------------------------===
+//
+// 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
+//
+// ===----------------------------------------------------------------------===
+
+struct AgentNameValidationRule: InputValidationRule {
+    
+    // MARK: Functions
+    
+#if swift(>=6.0)
+    func validate(_ input: String?) throws(InputValidationError) -> Bool {
+        try validate(input: input)
+    }
+#else
+    func validate(_ input: String?) throws -> Bool {
+        try validate(input: input)
+    }
+#endif
+    
+}
+
+// MARK: - Definitions
+
+extension InputValidationRule where Self == AgentNameValidationRule {
+    
+    // MARK: Constants
+    
+    /// A validation rule that checks whether an input is camel-cased or not.
+    static var agentName: Self { .init() }
+    
+}
+
+// MARK: - Helpers
+
+private extension AgentNameValidationRule {
+    
+    // MARK: Functions
+
+    /// Validates a given input.
+    ///
+    /// > note: This helper function would not be necessary when support for *Swift 5.10* is discontinued.
+    ///
+    /// - Parameter input: An input to be validated.
+    /// - Returns: A flag that indicates whether a given input has been validated or not.
+    /// - Throws: An error of type ``InputValidatorError`` in case the validation failed.
+    func validate(input: String?) throws -> Bool {
+        guard let input else {
+            return false
+        }
+        
+        guard input.fullyMatch(
+            pattern: .init(format: .Pattern.agentName)
+        ) else {
+            throw InputValidationError.inputNotAgentName
+        }
+        
+        return true
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension String.Pattern {
+    /// A regular expression pattern to match the user agent name input against.
+    static let agentName = "^([A-Z]([a-z]|[0-9])+)+$"
+}