test: concurrent strategy gaps (DetachTimeout expiry is untested, bound test can take 10 min to fail) #146

Two gaps in the concurrent strategy's tests, found by the independent verifier of #144. Neither is a runtime bug today.

1. No test catches a slot waiter that runs after its DetachTimeout. When a slot waiter's context ends, acquireConcurrencySlot logs "slot wait abandoned" and returns false (runtime.go#L1224-L1226). The verifier changed that branch to return true, so the waiter would run its handler anyway. The whole root test suite still passed, with the test from before test(concurrent): check the MaxConcurrent bound across the whole slot wait #144 and with the one after it.
2. TestConcurrentBoundedByMaxConcurrent can take 10 minutes to fail. Since test(concurrent): check the MaxConcurrent bound across the whole slot wait #144 it runs in a testing/synctest bubble. If a broken runtime leaves a goroutine blocked on a sync.Mutex, synctest.Wait (concurrent_test.go#L148) waits until the go test timeout (10 minutes by default) instead of failing at once. The test before test(concurrent): check the MaxConcurrent bound across the whole slot wait #144 failed the same broken runtime in 2 s. CI still reports the failure within its 30-minute job limit.

Possible fix

1. Add a test where a slot waiter's DetachTimeout expires while the slot stays held, then assert its handler never runs and the abandon is observable.
2. Decide whether the 10-minute worst case is acceptable. If it is not, put a bound around the bubble, for example a deadline in TestMain or a short -timeout for this package in CI.

Owner and trigger

The coder/chat maintainer triages this. Fix item 1 with the next change to the concurrent strategy.

Generated with xum • Model: anthropic:claude-opus-5-5 • Thinking: xhigh