Zulip stream type check

[apiraino]

The Zulip Message API has changed a little bit since we implemented our client. This patch:

• replaces "private" with "direct" for direct messages (see docs), no functional change.
• fixes/improves the direct vs. stream check we do when the triagebot server receives a message from the Zulip server. The previous check was not wrong per se but the correct check should be on the message type.
• Rewrote/fixed and (hopefully) clarified the documentation about testing Zulip stuff

tbh these changes are not really necessary but slightly improve the correctness of our client.

For anyone who has an opinion :)

[rustbot]

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[Urgau]

Looks okay to me, but I haven't tested it, so I will let others take a look otherwise I will merge in the next few days.

[apiraino]

thanks! Testing this is a bit tedious, but I will do that in the next couple of days

@rustbot author

[apiraino]

notice that I also removed the "unused" attribute introduced in 41a793d

[apiraino]

Hmm waiting on the Zulip folks to reply to a question because my tests seems to indicate that Zulip still sends the old payload.

If true, this patch would break everything so I'll put it into draft status to be sure it's not merged.