Slack: non-unfurl attachment content (title/text/fields) is dropped during message normalization #608

Summary

Slack messages from integrations (Sentry, PagerDuty, GitHub, most alerting tools) carry their real payload in attachments — title, text, fallback, and the fields array — and/or blocks. The Slack adapter only reads attachments to build link previews (from_url/original_url unfurls). Attachments that aren't link unfurls are discarded: their content appears nowhere on the normalized Message (msg.text is just the top-level text; msg.attachments is for files).

Impact

An alert that visibly renders a title, description, and key/value fields in Slack normalizes to a Message whose text is only the one-line summary. Everything consuming normalized messages inherits the blindness — thread.messages, toAiMessages, and the chat/ai tools (fetchMessages returns gutted alert messages to an agent). The only workaround is parsing msg.raw per platform, which defeats the normalization layer.

Related: #536 reports the same class of issue on Teams (file cards dropped during normalization).

Suggested fix

Fold non-unfurl attachment content into the normalized message — append fallback/title/text/fields into text/formatted, or expose a dedicated field (e.g. attachmentText). The adapter already iterates the attachments array during normalization.