fix(google-calendar): address review findings

- log an unknown stored zone once even when the tool description reads it first
- serialise the flow's token write with /calendar_disconnect under the flow gate
- a cancelled or expired pending flow is cleared, not re-sent or shown
- chat keys anchor with \z so a trailing newline is hashed
- a timeMax date includes that whole day (spec updated)
- dispose the flow's CancellationTokenSource, drop ConnectFlow.Token, Begin is a plain write
- GoogleCalendarOptions.IsConfigured replaces the host's copy of the rule; StoredToken is internal
- test BotSetupService's menu through DI, tokens absent from tool results
- Telegram.Tests csproj reference placed as in telegram-message-context; ChatHost passes calendar by name
- docs: per-event timeZone, both config sources for /calendar_*

hosts/Volcanic.AgenticCore.Telegram/GoogleCalendarSetup.cs

@@ -34,8 +34,7 @@ public static class GoogleCalendarSetup
     }
 
     /// <summary>True when both the client id and the secret are set, i.e. the extension will register its tool and commands.</summary>
-    public static bool IsActive(GoogleCalendarOptions? options) =>
-        options is not null && NonEmpty(options.ClientId) is not null && NonEmpty(options.ClientSecret) is not null;
+    public static bool IsActive(GoogleCalendarOptions? options) => options?.IsConfigured == true;
 
     /// <summary>The bot's command menu, with the calendar commands appended when the calendar is active.</summary>
     public static IReadOnlyList<BotCommand> MenuFor(IReadOnlyList<BotCommand> menu, GoogleCalendarOptions? options) =>