Assign one or more users to a chat and, optionally, set the PRIMARY assignee.

• Already-assigned users are skipped (idempotent).
• primaryAssigneeUserId, if provided, must be in userIds. It unsets any existing primary so a chat always has at most one primary. Re-sending the current primary is a no-op (no thread message).
• A system message is posted to the chat thread describing the change.

Security:

• The organisationId in the URL must match the organisation your JWT was issued for — requests for a different organisation fail with 401.
• Requires CRM_ADMIN, ACCOUNT_OWNER, or CRM_BOT role; other roles receive 403.