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>

This commit was merged in pull request #28.