refactor: extract remaining Telegram handlers to platform-neutral contracts

- Extract CreateSessionHandler, ListSessionsHandler, DeleteSessionHandler,
  ExportCalendarHandler, HandleRescheduleTimeInputHandler,
  HandleRescheduleVoteHandler to GmRelay.Shared
- Add IPlatformMessenger methods: SendScheduleAsync, UpdateScheduleAsync,
  SendGroupMessageAsync with actions, CreateThreadAsync, DeleteThreadAsync
- Rewrite Telegram Bot wrappers as thin adapters delegating to shared handlers
- Rewrite DiscordRescheduleVoteHandler to use shared HandleRescheduleVoteHandler
- Update UpdateRouter with explicit type aliases for ambiguous handler names
- Add contract and source-inspection tests for extracted handlers
- Bump version 3.1.1 → 3.2.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/GmRelay.Bot.Tests/Features/Sessions/ExportCalendar/ExportCalendarCommandContractTests.cs

@@ -0,0 +1,31 @@
+using GmRelay.Shared.Features.Sessions.ExportCalendar;
+using GmRelay.Shared.Platform;
+
+namespace GmRelay.Bot.Tests.Features.Sessions.ExportCalendar;
+
+public sealed class ExportCalendarCommandContractTests
+{
+    [Fact]
+    public void ExportCalendarCommand_ShouldExposePlatformNeutralContext()
+    {
+        AssertProperty<ExportCalendarCommand>("Group", typeof(PlatformGroup));
+        AssertProperty<ExportCalendarCommand>("User", typeof(PlatformUser));
+        AssertNoTelegramSpecificProperties<ExportCalendarCommand>();
+    }
+
+    private static void AssertProperty<T>(string name, Type expectedType)
+    {
+        var property = Assert.Single(typeof(T).GetProperties(), p => p.Name == name);
+        Assert.Equal(expectedType, property.PropertyType);
+    }
+
+    private static void AssertNoTelegramSpecificProperties<T>()
+    {
+        var names = typeof(T).GetProperties().Select(p => p.Name).ToArray();
+        Assert.DoesNotContain(names, name => name.Contains("Telegram", StringComparison.Ordinal));
+        Assert.DoesNotContain("ChatId", names);
+        Assert.DoesNotContain("MessageId", names);
+        Assert.DoesNotContain("TelegramUserId", names);
+        Assert.DoesNotContain("TelegramUsername", names);
+    }
+}