`_fetch_thread_context` and `_fetch_thread_parent_text` only read each
message's plain `text` field, so messages posted by apps (Alertmanager,
Grafana, PagerDuty, CI bots) — which carry their content in legacy
`attachments` or Block Kit `blocks` with an empty `text` — were dropped
entirely. When such a message *starts* a thread (e.g. an alert), a bot
mentioned mid-thread to investigate sees an empty thread and can only ask
"what should I investigate?".
Fall back to the existing `_extract_text_from_slack_blocks` and a new
`_extract_text_from_slack_attachments` helper when `text` is empty, so
app-posted alerts and notifications are visible in fetched thread history.
Adds TestThreadContextAppMessages (attachment-only, blocks-only, and
empty-message cases).