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.Shared/Features/Sessions/CreateSession/Wizard/IWizardDraftRepository.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace GmRelay.Shared.Features.Sessions.CreateSession.Wizard;
+
+/// <summary>
+/// Storage contract for wizard drafts. Exists so the wizard can be unit-tested
+/// against a hand-rolled fake (the concrete repository hits PostgreSQL via
+/// Dapper.AOT and is therefore unsuitable for fast in-process tests).
+/// </summary>
+public interface IWizardDraftRepository
+{
+    Task<WizardDraft?> GetActiveAsync(long chatId, int? messageThreadId, long ownerTelegramId, CancellationToken ct);
+
+    Task UpsertAsync(WizardDraft draft, CancellationToken ct);
+
+    Task DeleteAsync(Guid id, CancellationToken ct);
+
+    Task<int> DeleteExpiredAsync(CancellationToken ct);
+}

src/GmRelay.Shared/Features/Sessions/CreateSession/Wizard/WizardDraftRepository.cs

@@ -6,7 +6,7 @@ using Npgsql;
 
 namespace GmRelay.Shared.Features.Sessions.CreateSession.Wizard;
 
-public sealed class WizardDraftRepository(NpgsqlDataSource dataSource)
+public sealed class WizardDraftRepository(NpgsqlDataSource dataSource) : IWizardDraftRepository
 {
     public async Task<WizardDraft?> GetActiveAsync(
         long chatId, int? messageThreadId, long ownerTelegramId, CancellationToken ct)