Covers cluster C1-images (#69185, #32315, #66136):
- _slack_file_marker unit tests: typed markers per mimetype family,
hostile-filename sanitization (newlines/brackets can't fake context
structure).
- _render_message_text appends markers; a caption-less image post no
longer vanishes from thread context.
- Cold-start hydrate integration: prior-message images surface as
markers in channel_context; the thread root's image is downloaded,
delivered as media_urls, and upgrades message_type to PHOTO.
- Failure path: root-image download failure degrades to the marker,
never blocks the turn.
- Bounds: root delivery capped at _THREAD_ROOT_IMAGE_MAX; non-image
root attachments stay marker-only (no download).
- One-time delivery: active thread session skips the hydrate → no
re-download/re-delivery on later turns.
- Composition: the trigger's own event files still ride alongside a
delivered root image; Slack Connect stubs resolve via files.info;
the collector never issues its own conversations.replies call.
- Delta refresh (#23918 path): images in new replies past the watermark
surface as markers, with no root re-download.
A/B: 14 of 15 tests fail with the adapter fix reverted, all pass with
it applied.
Teknium
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