fix(bot,discord): allow 'no player limit' option in /newsession wizard

In the session creation wizard (Telegram + Discord), the Capacity step
only exposed waitlist on/off buttons. The 'no waitlist' button silently
advanced to the next step without setting MaxPlayers, so users who tried
to create a session with no player cap were blocked with
'Не заполнены поля: лимит мест'.

The DB contract and CreateSessionCommand already supported null
MaxPlayers (int?, ck_sessions_max_players check in V006), and the web
form already exposes 'Без лимита' as an empty InputNumber — only the
wizard flow was broken.

Changes:
- Add '♾ Без лимита' choice button to Capacity in shared
  WizardStepViewBuilder.BuildCapacity (Telegram) and to
  RenderCapacity / RenderPoolSlotCapacity in DiscordWizardStep (Discord).
- Add 'no_limit' branch to GameCreationWizard.ApplyCapacityChoice that
  sets MaxPlayers to null and advances to Visibility.
- Change GameCreationWizard.SetMaxPlayers signature from int to int? so
  the 'no limit' branch compiles.
- Change CreateSessionCommand builder in both Telegram and Discord
  submitters to take int? maxPlayers and drop the '?? 0' that would
  have turned null into 0 (violating the DB CHECK and the 'no limit'
  contract).
- In Discord BuildConfirmDescription, render '👥 Без лимита, waitlist
  вкл/выкл' when MaxPlayers is null (the previous code silently
  omitted the line).
- Expose BuildCommand as internal in both submitters and add
  InternalsVisibleTo('GmRelay.Bot.Tests') to the DiscordBot assembly
  for unit-test access.

Tests (9 new):
- WizardStepRenderTests.CapacityStep_HasWaitlistButtons — asserts the
  'Без лимита' button is present.
- GameCreationWizardStepTransitionsTests.NoLimitCapacityButton_… —
  asserts the choice advances to Visibility and leaves MaxPlayers null
  in the JSON draft.
- GameCreationWizardStepTransitionsTests.ChoiceCallback_AdvancesToExpectedStep —
  new Theory row for Capacity/no_limit.
- CreateSessionHandlerBuildCommandTests (new) — null/value propagation
  through the Telegram submitter's BuildCommand.
- DiscordWizardStepCapacityRenderTests (new) — 'Без лимита' button is
  rendered for both Capacity and PoolSlotCapacity, with the expected
  custom-id shape.
- DiscordWizardSubmitterBuildCommandTests (new) — null/value
  propagation through the Discord submitter's BuildCommand.

Closes #123

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

@@ -302,6 +302,7 @@ public sealed class GameCreationWizard
     {
         "waitlist:on" => (WizardStepNames.Visibility, SetWaitlist(p, true)),
         "waitlist:off" => (WizardStepNames.Visibility, SetWaitlist(p, false)),
+        "no_limit" => (WizardStepNames.Visibility, SetMaxPlayers(p, null)),
         _ => (null, "Неизвестный выбор"),
     };
 
@@ -416,7 +417,7 @@ public sealed class GameCreationWizard
     private static string? SetDurationMinutes(WizardPayload p, int? v) { p.DurationMinutes = v; return null; }
     private static string? SetScheduledAt(WizardPayload p, DateTimeOffset v)
     { p.Single ??= new WizardSingleInput(); p.Single.ScheduledAt = v; return null; }
-    private static string? SetMaxPlayers(WizardPayload p, int v)
+    private static string? SetMaxPlayers(WizardPayload p, int? v)
     { p.Single ??= new WizardSingleInput(); p.Single.MaxPlayers = v; return null; }
     private static string? SetWaitlist(WizardPayload p, bool v) { p.Waitlist = v; return null; }
     private static string? SetVisibility(WizardPayload p, WizardVisibility? v) { p.Visibility = v; return null; }

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

@@ -97,11 +97,12 @@ public static class WizardStepViewBuilder
         BackCancel());
 
     private static (string, IReadOnlyList<WizardAction>) BuildCapacity() => (
-        "👥 Введите лимит мест (1..50) одним числом.\nЗатем нажмите кнопку waitlist.",
+        "👥 Введите лимит мест (1..50) одним числом.\nЗатем нажмите кнопку waitlist. Или сразу «♾ Без лимита».",
         new List<WizardAction>
         {
             new("✅ Waitlist вкл", WizardCallbackData.Choice(WizardStepNames.Capacity, "waitlist:on"), WizardActionStyle.Success),
             new("❌ Без waitlist", WizardCallbackData.Choice(WizardStepNames.Capacity, "waitlist:off"), WizardActionStyle.Danger),
+            new("♾ Без лимита", WizardCallbackData.Choice(WizardStepNames.Capacity, "no_limit"), WizardActionStyle.Primary),
         });
 
     private static (string, IReadOnlyList<WizardAction>) BuildVisibility() => (