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.Bot/Features/Sessions/CreateSession/CreateSessionHandler.cs

@@ -170,7 +170,7 @@ public sealed class CreateSessionHandler
                 draft,
                 p,
                 new[] { p.Single?.ScheduledAt ?? default },
-                p.Single?.MaxPlayers ?? 0,
+                p.Single?.MaxPlayers,
                 isOneShot: true),
         };
     }
@@ -178,11 +178,11 @@ public sealed class CreateSessionHandler
     private static int MaxPlayersForPool(WizardPoolInput pool) =>
         pool.Slots.Count == 0 ? 0 : pool.Slots.Max(s => s.MaxPlayers);
 
-    private static CreateSessionCommand BuildCommand(
+    internal static CreateSessionCommand BuildCommand(
         WizardDraft draft,
         WizardPayload p,
         IReadOnlyList<DateTimeOffset> scheduledTimes,
-        int maxPlayers,
+        int? maxPlayers,
         bool isOneShot)
     {
         var user = new PlatformUser(