Bot-posted alerts (Honeycomb, PagerDuty, Datadog, GitHub bot, etc.) carry
their actionable content — section text, button URLs — in Block Kit
blocks, while the plain text field holds only the alert title.
_fetch_thread_context and _fetch_thread_parent_text only read
msg.get('text'), so that content never reached the agent.
Add a _render_message_text helper that merges top-level text with
readable block content, section/header/context text, actionable URLs,
and (folded in from #61261 during conflict resolution) legacy
attachment fields, and use it for thread-context and parent-text
rendering.
Salvaged from #29541.
Benjamin Ross
3865694cf9