test: no test covers a sync-dispatch slot waiter whose request ends #151

No test covers a DispatchSync slot waiter whose request context ends while it waits for a concurrency slot.

What happens

The independent verifier of #150 changed the abandon branch of acquireConcurrencySlot (runtime.go#L1242) to wait for the slot and then return true. So a waiter whose context ended would still run its handler. #150's new test catches that under DispatchDeferred, where the context is the DetachTimeout one. Under DispatchSync the same branch serves a waiter whose request context ends, for example when the platform's client gives up. With the change in place, the root suite still passed, so nothing pins the sync path.

Possible fix

Add a DispatchSync test where the only slot stays held and the waiter's request context is cancelled. Assert that its handler never runs and that the abandon is observable. Use the same testing/synctest pattern as TestConcurrentSlotWaiterAbandonedAtDetachTimeout.

Owner and trigger

The coder/chat maintainer triages this. Add the test with the next change to the concurrent strategy.

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