Update Zulip integration terminology and add per-thread topic mapping

Hi Team,

I’ve submitted a PR to update the Zulip integration in discourse/discourse:

This is part of the work tracked in the Zulip project:

The PR includes:

  • Terminology update: streamchannel, subjecttopic to match
    current Zulip terminology (Zulip renamed these in Zulip Server 10.0)
  • Per-thread topic mapping: when the topic field is left blank, the
    Discourse thread title is automatically used as the Zulip topic,
    mapping each Discourse thread to its own Zulip topic
  • Optional parameter support in channel.rb and edit-channel.gjs
  • Migration file updated

Would appreciate a review when you get a chance!

Thank you!