test(wizard): add wizard tests + refactor to IWizardDraftRepository

- Extract IWizardDraftRepository interface for testability (NSubstitute cannot
  mock sealed classes; the codebase uses fake-style doubles instead).
- Add step-transition, pool-slot, validation, cancel/back, and render-shape tests
  using FakeWizardDraftRepository and FakeWizardMessenger.
- Fix wizard payload persistence bug: HandleCallbackAsync and HandleTextAsync
  now call SavePayload after ApplyChoice/ApplyText mutations, so subsequent
  LoadPayload calls see the user's progress. Previously, local WizardPayload
  mutations were discarded and the wizard reset on every step.
- CommitCurrentPoolSlot now auto-creates a slot via EnsureCurrentPoolSlot when
  one is missing, so the PoolSlotCapacity → waitlist click is recoverable
  even if the user lands on the step without a slot.

src/GmRelay.Bot/Program.cs

@@ -72,7 +72,7 @@ builder.Services.AddSingleton<GmRelay.Shared.Features.Sessions.CreateSession.Cre
 builder.Services.AddSingleton<GmRelay.Bot.Features.Sessions.CreateSession.CreateSessionHandler>();
 
 // Wizard services (issue #111)
-builder.Services.AddSingleton<WizardDraftRepository>();
+builder.Services.AddSingleton<IWizardDraftRepository, WizardDraftRepository>();
 builder.Services.AddSingleton<ITelegramWizardMessenger, TelegramWizardMessenger>();
 builder.Services.AddSingleton<GameCreationWizard>();
 builder.Services.AddSingleton<IScheduleMessageUpdateLock, ScheduleMessageUpdateLock>();