flaky: TestSlackPostNeverSleepsPastContextDeadline fails under full-suite -race load #39

adapters/slack TestSlackPostNeverSleepsPastContextDeadline intermittently fails on unmodified main (observed at 20e55d6, 1 of 3 full-suite runs) when the whole workspace runs under GOFLAGS=-race:

ratelimit_hardening_test.go:257: err = slack: chat.postMessage request: Post "http://127.0.0.1:.../chat.postMessage": context deadline exceeded, want *slack.RateLimited (deadline-bounded, not slept off)

It passes reliably in isolation (go test -race -count=6 ./adapters/slack/); the failure only appears under heavy parallel package load, where the plain HTTP POST to the local httptest server itself exceeds the test's short context deadline before the rate-limit path is exercised. The deadline budget likely needs headroom (or the test should distinguish "transport never completed" from "slept past deadline").

Observed while validating #38 (which does not touch the slack adapter); reproduced independently on main at 20e55d6.

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