Support 'direct' message type for private messages #8

Job	Run time
Debian 11 (Python 3.9, backend) (Zulip refs/tags/4.0)	4m 53s
Debian 12 (Python 3.11, backend + documentation) (Zulip refs/tags/7.0)	5m 57s
Ubuntu 24.04 (Python 3.12, backend) (Zulip refs/tags/8.5)	7m 21s
Ubuntu 22.04 (Python 3.10, backend) (Zulip refs/pull/36159/head)	6m 18s
	24m 29s

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