feat: async chain end to end

[Theauxm]

Summary

50 sample trains migrated to the new Junctions() shape from Trax.Core. Mechanical edit: return type wraps in Task<Either<Exception, ...>>, body appends .Resolve(), adds using LanguageExt; where missing.

Sample apps (GameServer, EnergyHub, ChatService, JobHunt, ContentShield, Flowthru.Spaceflights, TestRunner, ApiAudit) all build clean and most test suites pass.

Test plan

• All sample app projects build clean, zero warnings
• Unit + E2E suites: 289/294 sample tests pass
• dotnet csharpier check . passes
• 5 known failures in Trax.Samples.GameServer.E2E that look unrelated to the chain refactor — calling out for visibility:
  • BanPlayer_WithAdminKey_Succeeds and BanPlayer_RunMutation_ExecutesInline fail with Not authorized despite the admin key carrying both Admin and Player roles. The inverse test (BanPlayer_WithPlayerKey_Returns403) passes, so the auth pipeline reaches the right verdict on the deny path. The auth packages have no chain code (grep -rn "Chain\|Junctions\|Resolve" Trax.Api.Auth* → 0 hits).
  • OnTrainStarted_ReceivesEvent, OnTrainCompleted_ReceivesEvent, OnTrainFailed_NoEventForNonBroadcastTrain all fail at WebSocketClient.ConnectAsync with Incomplete handshake, status code: 200 — TestHost is returning 200 instead of 101 on the WS upgrade. Independent of the chain.

Migration

See Trax.Docs migration guide for the consumer-facing pattern.

[traxsharp]

This PR is included in version 1.21.0