Multiple overlapping Slack attachment improvements:
1. Upload retry with backoff on transient errors (429, 5xx, connection
reset, rate_limited, service unavailable). New _is_retryable_upload_error
helper covers three upload paths: _upload_file, send_video,
send_document. Up to 3 attempts with 1.5s * attempt backoff.
2. Thread participation tracking: successful file uploads now add the
thread_ts to _bot_message_ts, mirroring how text replies are tracked.
This lets follow-up thread messages auto-trigger the bot (same
engagement rules as replied threads).
3. Thread metadata preservation in the image redirect-guard fallback
(send_image → send text fallback) and in two gateway.run.py send
paths (image + document fallback calls).
4. HTML response rejection in _download_slack_file_bytes. Parallels
the existing check in _download_slack_file. Guards against Slack
returning a sign-in / redirect page as document bytes when scopes
are missing, so the agent doesn't get HTML-as-a-PDF.
5. File lifecycle event acks (file_shared / file_created / file_change).
These events arrive around snippet uploads. Acking them silences the
slack_bolt 'Unhandled request' 404 warnings without changing behavior.
6. Post-loop message type classification so a mixed image+document upload
classifies as PHOTO (or VOICE if no image), falling back to DOCUMENT.
Previously, the per-file classification in the inbound loop could be
overwritten unpredictably.
7. Expanded text-inject whitelist in inbound document handling to cover
.csv, .json, .xml, .yaml, .yml, .toml, .ini, .cfg (up to 100KB) so
snippets and config files are directly visible to the agent, not just
cached as opaque uploads. Paired with new MIME entries in
SUPPORTED_DOCUMENT_TYPES in base.py.
Squashed from two commits in #11819 so the single commit carries the
contributor's GitHub attribution (the original commits were authored
under a local dev hostname).
e818ec520a