Add internal_article to ai_agent content_source content_type enum (#664)

Companion to intercom/intercom#574680, which returns internal articles
in the conversation ai_agent.content_sources list on Preview.


Claude-Session: https://claude.ai/code/session_01MjNW7SReqMM7QTqkuoiNye

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: SakibulMowla

descriptions/0/api.intercom.io.yaml

@@ -32062,6 +32062,7 @@ components:
           - external_content
           - content_snippet
           - workflow_connector_action
+          - internal_article
         url:
           type: string
           description: The internal URL linking to the content source for teammates.