fix: expose actionable leaderboard score errors

[JoelFerrando]

Summary

• map Android SIGN_IN_REQUIRED by ApiException.statusCode, without parsing localized messages
• return a dedicated code when the player declines the Android friends-list resolution
• map GameKit GKError.notAuthenticated and GKError.cancelled to stable codes
• expose LeaderboardScoresErrorCode constants through the Dart package
• document the recoverable cases

Why

loadLeaderboardScores currently collapses native failures into failed_to_load_leaderboard_scores. Callers cannot distinguish an expired sign-in session, a declined friends-list request, or a canceled GameKit operation from an unknown service failure.

This supersedes only the leaderboard portion accidentally included in #238. It deliberately avoids localized-message matching and does not classify GKError.cancelled as a consent error.

Verification

• Android native unit tests cover SIGN_IN_REQUIRED, DEVELOPER_ERROR, and non-ApiException failures: :games_services:testDebugUnitTest passes
• physical V2310 / Android 15, using an isolated registered test credential:
  • authenticated global leaderboard load completed successfully through Dart → MethodChannel → Kotlin → Play Games
  • rejecting the real Play Games friends-list consent sheet returned friends_list_access_denied with Player declined friends list access
• an unregistered dev build returns Play Games status 10 and the plugin preserves it as failed_to_load_leaderboard_scores; this verifies that configuration/developer failures are not misclassified as authentication-required
• dart analyze in games_services_platform_interface: clean
• flutter analyze --no-fatal-warnings in games_services: only the existing development path dependency warning
• Android debug APK compiles successfully through the consumer app

Still to verify before marking ready:

• compile and exercise the GameKit changes on macOS/Xcode. We do not currently have an Apple test environment available; this is planned for next week. No iOS behavior is being claimed as verified yet.

[JoelFerrando]

Android verification is now complete: an authenticated global load succeeded on a physical device, and rejecting the real Play Games friends-list consent sheet returned friends_list_access_denied as intended. I am keeping this as a draft because I do not currently have macOS/Xcode available to compile and exercise the GameKit changes; I expect to have access next week and do not want to claim the iOS side is verified before then.