Defined the InputValidationRule protocol in the library target.

Sources/DiscogsService/Internal/Errors/InputValidationError.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
+//
+// ===----------------------------------------------------------------------===
+
+/// A representation of all the possible validation error that could be thrown while validating an input.
+enum InputValidationError: Error {
+    /// An input is empty.
+    case inputIsEmpty
+    /// An input is nil.
+    case inputIsNil
+}

Sources/DiscogsService/Internal/Protocols/InputValidationRule.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
+//
+// ===----------------------------------------------------------------------===
+
+/// A protocol that defines an input validation rule to be applied to an input by the ``ValidateInputUseCase`` use case.
+protocol InputValidationRule {
+
+    // MARK: Functions
+
+#if swift(>=6.0)
+    /// Validates a given input against a validation rule.
+    /// - Parameter input: An input to be validated.
+    /// - Returns: A flag that indicates whether an input has been validated or not.
+    /// - Throws: An error of type ``InputValidationError`` in case a given input failed a validation.
+    @discardableResult func validate(_ input: String?) throws(InputValidationError) -> Bool
+#else
+    /// Validates a given input against a validation rule.
+    /// - Parameter input: An input to be validated.
+    /// - Returns: A flag that indicates whether an input has been validated or not.
+    /// - Throws: An error of type ``InputValidationError`` in case a given input failed a validation.
+    @discardableResult func validate(_ input: String?) throws -> Bool
+#endif
+    
+}