fix(bot): IsComplete must not flag null MaxPlayers as missing (no-limit)

After 3.9.6 fixed long-polling, the bot finally reaches the final
'✅ Создать' step. Users pressing '♾ Без лимита' on the Capacity step
get a valid payload where Single.MaxPlayers = null (the legitimate
no-limit choice from GameCreationWizard.ApplyCapacityChoice 'no_limit'),
but CreateSessionHandler.IsComplete then reports 'лимит мест' as
missing, blocking session creation.

This regression existed since 3.9.3 (when 'no_limit' was added) but
stayed invisible because 3.9.4 and 3.9.5 never reached SubmitDraft
(libgssapi-krb5 missing → long-polling hung). Once 3.9.6 restored
polling, the bug surfaced immediately.

Fix: drop the null-MaxPlayers check from IsComplete for Single type.
Null is a valid 'no limit' state and must pass through to BuildCommands
→ shared handler, which already accepts null MaxPlayers correctly.

Closes #131.

Bump version 3.9.6 -> 3.9.7

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

@@ -229,7 +229,11 @@ public sealed class CreateSessionHandler
         if (p.Type == WizardCreationType.Single)
         {
             if (p.Single?.ScheduledAt is null) missingFields.Add("дата/время");
-            if (p.Single?.MaxPlayers is null) missingFields.Add("лимит мест");
+            // MaxPlayers = null is a valid "♾ Без лимита" choice
+            // (see GameCreationWizard.ApplyCapacityChoice "no_limit"). Only a
+            // missing wizard step on the numeric-input path is a real defect;
+            // a non-null 0 also blocks (zero players is never what the user
+            // asked for). Treat null as "no limit" and accept it.
         }
         else
         {