* fix: surface Slack thread errors
Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
* fix: don't set failure_reply_posted on Slack preprocessing errors
The preprocessing error handler was setting failure_reply_posted=True,
the same idempotency flag handle_run_completion checks to suppress
duplicate run-failure replies. Since preprocessing failures happen
before any run exists but the flag persists on the thread, a subsequent
run failure on the same thread would be silently ignored.
The preprocessing handler already posts its own Slack reply, so the
run-completion idempotency flag should not be set here.
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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
