Support using "direct" as message type for private messages (#779)

[sujanyd]

This PR adds support for using "direct" as the message type when sending private messages via the Zulip Python API CLI.

Key Changes:
• CLI now accepts "direct" as a valid message type (--type direct).
• Automatically translates "direct" → "private" for servers with API feature level < 174 (older servers).
• Keeps "direct" unchanged for newer servers (feature level ≥ 174).
• Logging updated to show the actual message type being sent.

Testing:
• Verified sending messages with "direct" and "private".
• Checked correct fallback for older servers.

Related Issue:
Fixes #779

[zulipbot]

Hello @sujanyd, it seems like you have referenced #779 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #779..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

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