Fix ArgumentNullException in EchoBot when joining via short Teams meeting URL

[Copilot]

JoinInfo.ParseJoinURL returns an empty ChatInfo (null ThreadId) for short URLs (https://teams.microsoft.com/meet/<id>?p=<passcode>). BotService used ChatInfo.ThreadId directly as a ConcurrentDictionary key, throwing ArgumentNullException on every short URL join attempt.

Changes — BotService.cs

• JoinCallAsync: Guard duplicate-call check against null ThreadId; short URL joins skip the pre-join check and proceed directly to AddAsync
• CallsOnUpdated: Use call.Resource.ChatInfo?.ThreadId ?? call.Id as the tracking key so handlers are always stored/removed with a valid non-null key

// Before — throws ArgumentNullException when ThreadId is null
if (!this.CallHandlers.TryGetValue(joinParams.ChatInfo.ThreadId, out CallHandler? call))

// After — safe for short URL joins where ThreadId is unknown pre-join
var threadId = joinParams.ChatInfo.ThreadId;
if (!string.IsNullOrEmpty(threadId) && this.CallHandlers.TryGetValue(threadId, out CallHandler? _))
{
    throw new Exception("Call has already been added");
}

// CallsOnUpdated — fall back to call.Id when ChatInfo.ThreadId is absent
var threadId = call.Resource.ChatInfo?.ThreadId ?? call.Id;

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