Implemented the ValidateInputUseCase use case in the library target.

Sources/DiscogsService/Internal/Use Cases/ValidateInputUseCase.swift

@@ -0,0 +1,53 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 use case that validates an input against a set of validation rules.
+struct ValidateInputUseCase {
+    
+    // MARK: Properties
+    
+    /// A list of validation rules to match an input against.
+    private let rules: [any InputValidationRule]
+    
+    // MARK: Initializers
+    
+    /// Initializes this use case.
+    /// - Parameter rules: A list of validation rules to match an input against.
+    init(rules: any InputValidationRule...) {
+        self.rules = rules
+    }
+    
+    // MARK: Functions
+    
+#if swift(>=6.0)
+    /// Validates an input against a set of validation rules.
+    /// - Parameter input: An input to be validated against a set of rules, if any.
+    /// - Throws: An error of type ``InputValidationError`` in case an input failed any validation.
+    func callAsFunction(_ input: String?) throws(InputValidationError) {
+        for rule in rules {
+            try rule.validate(input)
+        }
+    }
+#else
+    /// Validates an input against a set of validation rules.
+    /// - Parameter input: An input to be validated against a set of rules, if any.
+    /// - Throws: An error of type ``InputValidationError`` in case an input failed any validation.
+    func callAsFunction(_ input: String?) throws {
+        for rule in rules {
+            try rule.validate(input)
+        }
+    }
+#endif
+
+}

Tests/DiscogsService/Cases/Internal/Use Cases/ValidateInputUesCaseTests.swift

@@ -0,0 +1,97 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 Testing
+
+@testable import DiscogsService
+
+@Suite("Validate Input Use Cases")
+struct ValidateInputUseCaseTests {
+    
+    // MARK: Functions
+    
+#if swift(>=6.2)
+    @Test(arguments: zip(
+        Input.inputsToValidate,
+        Output.inputsToValidate
+    )) func `validate`(
+        input: String?,
+        expects error: InputValidationError?
+    ) async throws {
+        try assertValidate(
+            input: input,
+            expects: error
+        )
+    }
+#else
+    @Test("validate", arguments: zip(
+        Input.inputsToValidate,
+        Output.inputsToValidate
+    )) func validate(
+        input: String?,
+        expects error: InputValidationError?
+    ) async throws {
+        try assertValidate(
+            input: input,
+            expects: error
+        )
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension ValidateInputUseCaseTests {
+    
+    // MARK: Functions
+    
+    /// Asserts an input validation of a ``ValidateInputUseCase`` use case.
+    /// - Parameters:
+    ///   - input: An input to validate, if any.
+    ///   - error: An expected error, if any.
+    /// - Throws: An error of type ``InputValidationError`` in case of an unexpected test case scenario.
+    func assertValidate(
+        input: String?,
+        expects error: InputValidationError?
+    ) throws {
+        // GIVEN
+        let validate = ValidateInputUseCase(rules: .notNil, .notEmpty)
+        
+        // WHEN
+        // THEN
+        if let error {
+            #expect(throws: error) {
+                try validate(input)
+            }
+        } else {
+            #expect(throws: Never.self) {
+                try validate(input)
+            }
+        }
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension Input {
+    /// A list of inputs to validate against a set of validation rules.
+    static let inputsToValidate: [String?] = [nil, .empty, "SomeInput"]
+}
+
+private extension Output {
+    /// A list of expected input validation errors to be thrown (if necessary).
+    static let inputsToValidate: [InputValidationError?] = [.inputIsNil, .inputIsEmpty, nil]
+}

Tests/DiscogsService/Cases/Public/Middlewares/UserAgentMiddlewareTests.swift

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+// ===----------------------------------------------------------------------===
+//
+// 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 DiscogsService
+import Testing
+
+@Suite("User Agent Middleware", .tags(.middleware))
+struct UserAgentMiddlewareTests {
+
+//    @Test func <#test function name#>() async throws {
+//        // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+//    }
+
+}