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

tests/GmRelay.Bot.Tests/Features/Sessions/CreateSession/Wizard/WizardStepRenderTests.cs

@@ -76,6 +76,7 @@ public sealed class WizardStepRenderTests
         var labels = ButtonLabels(kb);
         Assert.Contains(labels, l => l.Contains("Waitlist вкл", StringComparison.Ordinal));
         Assert.Contains(labels, l => l.Contains("Без waitlist", StringComparison.Ordinal));
+        Assert.Contains(labels, l => l.Contains("Без лимита", StringComparison.Ordinal));
     }
 
     [Fact]