Slack Web API calls return `SlackResponse`/`AsyncSlackResponse`, which are
mapping-like but not `dict` subclasses, so every `isinstance(resp, dict)`
gate took its "unexpected shape" branch at runtime: user and channel names
collapsed to raw IDs, every user resolved as a non-bot (defeating the
allow_bots loop guard), ephemeral replies were reported as failures, and
uploads/caption fallbacks lost their message_id.
Normalize responses through a single `_slack_response_payload()` helper
(dict passes through, SDK response yields `.data`, anything else yields
`{}` so callers keep their fallbacks) and use it at every call site.
Existing Slack tests injected plain dicts, which is why the defect was
invisible; the new tests run each behavioral case against a real
`AsyncSlackResponse` as well.
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