invoke_acp_agent had no timeout anywhere in its call path, and
ACPAgentConfig had no timeout field. If the ACP agent subprocess answers
initialize/new_session correctly but then hangs inside prompt(), the tool
call - and therefore the whole agent turn - blocks indefinitely, with the
child process left running. MCP stdio servers already guard against this
class of hang via tool_call_timeout; ACP agent invocations had no
equivalent.
Add ACPAgentConfig.timeout_seconds (default 1800, ge=1), mirroring the
shape/default of subagents.timeout_seconds, and wrap the conn.prompt()
call in asyncio.wait_for(). On TimeoutError, return a clear error instead
of hanging; exiting the spawn_agent_process context block triggers the
ACP library's own graceful-then-forceful subprocess cleanup, so the hung
process is actually terminated.
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