Several fixes and optimizations all over the library (#28)

This PR contains the work done to cleans up the library after the Swift 6.2 migration: drops the legacy conditional-compilation paths, hardens the OpenAPI specification, removes boilerplate from the live client, and makes the live test suite resilient to changes in the upstream service data.

* Swift 6.2 baseline
  * Adopted swift-tools-version: 6.2, which allowed removing every #if swift(>=6.0) / #if swift(>=6.2) branch and natural-language test names are now the only code paths.
  * `AmiiboClient` client now refines `Sendable`, and both `AmiiboLiveClient` and `AmiiboService` conform to it, so instances can be shared across concurrency domains.

* Live client
  * Extracted a generic `perform(_:)` helper that wraps every generated API call and funnels transport errors through the existing error mapper — replaces six repeated do/catch blocks and lets the private fetch* methods use typed throws end-to-end.
  * Added the internal `KeyNamePayload` protocol and a generic makeModels(from:), collapsing the four near-identical mapping/sorting blocks for amiibo series, types, game characters and game series into one.
  * The server URL is now resolved once into a static let constant instead of on every initialization.
  * Improved error mapping: URLError.cancelled → .cancelled, .cannotParseResponse → .decoding, and .dataNotAllowed, .internationalRoamingOff, .secureConnectionFailed now map to .notAvailable.
  * Simplified the Amiibo.Platform initializer ([Amiibo+Platform.swift](vscode-webview://1m6rk4uhpaukb4hrbp3j54p77iq693fq2ql7o9oup2pca1omf7f6/Sources/AmiiboService/Public/Models/Amiibo/Amiibo+Platform.swift)) using optional chaining and ?? [] instead of nested guards and immediately-invoked closures.
  * Set the en_US_POSIX locale on both fixed-format date formatters so the user's locale or 12/24-hour setting can no longer break parsing.

* OpenAPI specification
  * Replaced the shared Tuple schema with explicit per-type schemas (AmiiboSeries, AmiiboType, GameCharacter, GameSeries), so the generated code exposes real properties instead of allOf wrappers with .value1 accessors.
  * Extracted named list schemas (AmiiboList, AmiiboSeriesList, …) for the wrapper payloads, which turns the generated .case2 enum cases into readable .AmiiboSeriesList cases.
  * Added validation patterns and length bounds to the id, head, tail and key query parameters, and collapsed the redundant pattern/minLength/maxLength triplets on head/tail into ^[0-9a-fA-F]{8}$.
  * Extracted the shared InternalServerError response, replacing the five inline 500 descriptions.
  * Documented the two non-standard date/time decoding behaviours (date-only strings for release dates, offset-less timestamps for lastUpdated) and why the optional amiibo wrapper property must stay optional.
  * Corrected the info.version value from v1.0.0 to 1.0.0.

* ⚠️ Breaking changes
  *  Minimum Swift version raised from 5.10 to 6.2.
  *  `AmiiboServiceError.unknown` now carries a String description of the underlying error.
  *  `AmiiboClient` requires Sendable conformance, so existing custom mock clients must be Sendable.

Reviewed-on: #28
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

README.md

@@ -13,7 +13,7 @@ To use this library, add it as a dependency in the `Package.swift` file:
 let package = Package(
     // name, platforms, products, etc.
     dependencies: [
-        .package(url: "https://github.com/rock-n-code/amiibo-service", from: "1.4.1"),
+        .package(url: "https://github.com/rock-n-code/amiibo-service", from: "1.4.2"),
         // other dependencies
     ],
     targets: [
@@ -31,7 +31,7 @@ let package = Package(
 It is also possible to use this library with your app in Xcode by adding it as a dependency in your Xcode project.
 
 > [!IMPORTANT]
-> Swift 5.10 or higher is required in order to build this library.
+> Swift 6.2 or higher is required in order to build this library.
 
 ## Usage
 
@@ -83,7 +83,7 @@ let service = AmiiboService(
 
 ## Testing
 
-The `AmiiboClient` protocol enables creating custom mock clients for unit testing without network calls. Conform to `AmiiboClient` and inject it into `AmiiboService` via its `init(client:)` initializer:
+The `AmiiboClient` protocol enables creating custom mock clients for unit testing without network calls. Conform to `AmiiboClient` and inject it into `AmiiboService` via its `init(client:)` initializer. Since `AmiiboClient` refines `Sendable`, conforming types must be safe to share across concurrency domains:
 
 ```swift
 import AmiiboService
@@ -104,6 +104,16 @@ struct MyMockClient: AmiiboClient {
 let service = AmiiboService(client: MyMockClient())
 ```
 
+### Running the test suite
+
+The unit tests based on a mock client run offline by default. The tests against the live service are skipped unless the `AMIIBO_LIVE_TESTS` environment variable is set to `1`, either in the environment when testing from the command line:
+
+```shell
+AMIIBO_LIVE_TESTS=1 swift test
+```
+
+or in the `Test` action of the scheme when testing from Xcode.
+
 ## Documentation
 
 Please refer to the [online documentation](https://rock-n-code.github.io/amiibo-service/documentation/amiiboservice/) for further information about this library.