Arranged the folder structure of the library and test targets in the Package.

Sources/AmiiboService/Internal/Extensions/DateFormatter+Properties.swift

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+//===----------------------------------------------------------------------===
+//
+// This source file is part of the AmiiboService open source project
+//
+// Copyright (c) 2024-2025 Röck+Cöde VoF. and the AmiiboAPI project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of AmiiboAPI project authors
+//
+//===----------------------------------------------------------------------===
+
+import Foundation
+
+extension DateFormatter {
+    
+    // MARK: Properties
+    
+    /// An ISO timestamp formatter.
+    ///
+    /// This formatter implements the `yyyy-MM-dd'T'HH:mm:ss.SSSSSS` custom date format.
+    /// Within the context of this library, this formatter is solely used to decode a date formatted as a timestamp that is returned by the ``AmiiboService/getLastUpdated()`` function.
+    static var isoTimestamp: DateFormatter {
+        let formatter = DateFormatter()
+        
+        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSSSS"
+        formatter.timeZone = .init(secondsFromGMT: 0)
+        
+        return formatter
+    }
+    
+}

Sources/AmiiboService/Internal/Protocols/APIClient.swift

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+//===----------------------------------------------------------------------===
+//
+// This source file is part of the AmiiboService open source project
+//
+// Copyright (c) 2024-2025 Röck+Cöde VoF. and the AmiiboAPI project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of AmiiboAPI project authors
+//
+//===----------------------------------------------------------------------===
+
+import Foundation
+
+/// A protocol that defines API clients containing all available endpoints to interact with.
+protocol APIClient {
+    
+    // MARK: Functions
+    
+    /// Gets a list of amiibo items based on a given filter.
+    /// - Parameter filter: A filter to remove unwanted items from the result.
+    /// - Returns: A list of filtered amiibo items.
+    /// - Throws: An ``AmiiboServiceError`` error in case some issue is encountered while generating the result.
+    func getAmiibos(by filter: AmiiboFilter) async throws(AmiiboServiceError) -> [Amiibo]
+    
+    /// Gets a list of amiibo series based on a given filter.
+    /// - Parameter filter: A filter to remove unwanted items from the result.
+    /// - Returns: A list of filtered amiibo series.
+    /// - Throws: An ``AmiiboServiceError`` error in case some issue is encountered while generating the result.
+    func getAmiiboSeries(by filter: AmiiboSeriesFilter) async throws(AmiiboServiceError) -> [AmiiboSeries]
+    
+    /// Gets a list of amiibo types based on a given filter.
+    /// - Parameter filter: A filter to remove unwanted items from the result.
+    /// - Returns: A list of filtered amiibo types.
+    /// - Throws: An ``AmiiboServiceError`` error in case some issue is encountered while generating the result.
+    func getAmiiboTypes(by filter: AmiiboTypeFilter) async throws(AmiiboServiceError) -> [AmiiboType]
+    
+    /// Gets a list of game characters based on a given filter.
+    /// - Parameter filter: A filter to remove unwanted items from the result.
+    /// - Returns: A list of filtered game characters.
+    /// - Throws: An ``AmiiboServiceError`` error in case some issue is encountered while generating the result.
+    func getGameCharacters(by filter: GameCharacterFilter) async throws(AmiiboServiceError) -> [GameCharacter]
+    
+    /// Gets a list of game series based on a given filter.
+    /// - Parameter filter: A filter to remove unwanted items from the result.
+    /// - Returns: A list of filtered game series.
+    /// - Throws: An ``AmiiboServiceError`` error in case some issue is encountered while generating the result.
+    func getGameSeries(by filter: GameSeriesFilter) async throws(AmiiboServiceError) -> [GameSeries]
+    
+    /// Gets the date when the data was last updated.
+    /// - Returns: A last updated date.
+    /// - Throws: An ``AmiiboServiceError`` error in case some issue is encountered while generating the result.
+    func getLastUpdated() async throws(AmiiboServiceError) -> Date
+    
+}

Sources/AmiiboService/Internal/Protocols/KeyNameFilter.swift

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+//===----------------------------------------------------------------------===
+//
+// This source file is part of the AmiiboService open source project
+//
+// Copyright (c) 2024-2025 Röck+Cöde VoF. and the AmiiboAPI project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of AmiiboAPI project authors
+//
+//===----------------------------------------------------------------------===
+
+/// A protocol that defines filters that might contain `key` and/or `name` values.
+protocol KeyNameFilter {
+    
+    // MARK: Properties
+    
+    /// A key to use for filtering, if any.
+    var key: String? { get }
+    
+    /// A name to use for filtering, if any.
+    var name: String? { get }
+    
+    // MARK: Initializers
+    
+    /// Initializes this filter without key or name values.
+    init()
+
+    /// Initializes this filter with a key value.
+    /// - Parameter key: A key to use for filtering.
+    init(key: String)
+    
+    /// Initializes this filter with a name value.
+    /// - Parameter name: A name to use for filtering.
+    init(name: String)
+    
+}

Sources/AmiiboService/Internal/Protocols/KeyNameModel.swift

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+//===----------------------------------------------------------------------===
+//
+// This source file is part of the AmiiboService open source project
+//
+// Copyright (c) 2024-2025 Röck+Cöde VoF. and the AmiiboAPI project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of AmiiboAPI project authors
+//
+//===----------------------------------------------------------------------===
+
+/// A protocol that defines decodable models containing the `key` and `name` properties.
+protocol KeyNameModel: Sendable {
+    
+    // MARK: Properties
+    
+    /// A key.
+    var key: String { get }
+    
+    /// A name.
+    var name: String { get }
+    
+    // MARK: Initializers
+    
+    /// Initializes this model from a given payload.
+    /// - Parameter payload: A payload that contains the values for the model.
+    init(_ payload: Components.Schemas.Tuple)
+    
+}

Sources/AmiiboService/Internal/Transcoders/ISOTimestampTranscoder.swift

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+//===----------------------------------------------------------------------===
+//
+// This source file is part of the AmiiboService open source project
+//
+// Copyright (c) 2024-2025 Röck+Cöde VoF. and the AmiiboAPI project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of AmiiboAPI project authors
+//
+//===----------------------------------------------------------------------===
+
+import Foundation
+import OpenAPIRuntime
+
+/// A type that allows the decoding and encoding of ISO timestamp dates, defined by the `yyyy-MM-dd'T'HH:mm:ss.SSSSSS` custom date format.
+struct ISOTimestampTranscoder {
+    
+    // MARK: Properties
+    
+    /// A formatter to use to decode and encode ISO timestamps dates.
+    private let dateFormatter: DateFormatter = .isoTimestamp
+    
+}
+
+ // MARK: - DateTranscoder
+
+extension ISOTimestampTranscoder: DateTranscoder {
+    
+    // MARK: Functions
+    
+    func encode(_ date: Date) throws -> String {
+        dateFormatter.string(from: date)
+    }
+    
+    func decode(_ string: String) throws -> Date {
+        dateFormatter.date(from: string) ?? .init()
+    }
+    
+}