Missing `default` alert type

[hds]

When leaving a bot running with an open event stream, I eventually (after a few days) get an event with "type": "shutdown" and "alertType":"default". This causes a JSON decoding error to be returned from WebexEventStream::next.

Here's an example of such a case:

{
  "id": "a39cdd93-e1bb-4361-a59f-449010--omitted--",
  "type": "shutdown",
  "timestamp": 0,
  "trackingId": "NA_4b40e577-3b56-4faa-ba5c-e4d1--omitted--",
  "alertType": "default",
  "sequenceNumber": 45,
  "filterMessage": false,
  "wsWriteTimestamp": 1769278479644
}

My guess is that it would be sufficient to add another variant to the AlertType enum.

webex-rust/src/types.rs

Lines 1022 to 1030 in f132301

	pub enum AlertType {
	/// This event won't ever generate an alert (?)
	#[default]
	None,
	/// This event will always generate an alert (?)
	Full,
	/// okay, no idea...
	Visual,
	}

I don't know if that type field should be included (optional I suppose) so that a shutdown can be detected and an appropriate error returned. I don't have much experience with the event stream, I've only used it through the webex crate, and I can't find it's documentation on developer.webex.com, but maybe I'm just looking in the wrong place.

I'd be happy to submit a fix if I could get a bit of guidance on what the right fix is in this case.