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/Features/Sessions/CreateSession/Wizard/WizardDraftCleanupService.cs

@@ -11,11 +11,11 @@ public sealed class WizardDraftCleanupService : BackgroundService
 {
     private static readonly TimeSpan TickInterval = TimeSpan.FromMinutes(1);
 
-    private readonly WizardDraftRepository _drafts;
+    private readonly IWizardDraftRepository _drafts;
     private readonly ILogger<WizardDraftCleanupService> _log;
 
     public WizardDraftCleanupService(
-        WizardDraftRepository drafts,
+        IWizardDraftRepository drafts,
         ILogger<WizardDraftCleanupService> log)
     {
         _drafts = drafts;