* fix(acp): fail fast when an ACP agent exits and drain its stderr
An ACP agent that exited before answering (missing credentials, a wrong
argument) left invoke_acp_agent waiting for the whole timeout_seconds
budget (30 minutes by default) and then suggested raising the timeout.
The pinned SDK never fails pending requests when the agent's stdout
reaches EOF, so nothing noticed the process was gone.
The SDK also opens the agent's stderr as a pipe and never reads it. An
agent that logged more than asyncio's pipe buffering absorbs blocked on
its next stderr write and stalled the invocation the same way.
Race the ACP exchange against the agent process: when it exits (or a
newer SDK reports the connection closed), allow a short grace for
responses still buffered in the pipe, then report the exit code. Drain
stderr continuously into a bounded tail that is logged on failure and
kept out of model-visible text.
* fix(acp): log the full stderr tail on timeout and pin tail retention
Log the timeout tail after the SDK context has terminated and reaped the
agent, so lines still in flight when it stalled are included. Make the
exit test write more stderr than the retained tail, so keeping the start
of the stream instead of its end fails the assertion on the fatal line.
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