test(acp): add stubborn process mode

Author: loong10k

src/test/resources/fake-acp-agent.py

@@ -6,15 +6,20 @@
   --slow-prompt             delay prompt completion so a second turn can race
   --exit-after-initialize   exit immediately after initialize succeeds
   --slow-initialize         delay initialize so close/connect can race
+  --ignore-term             ignore SIGTERM so close must escalate
   --malformed-on-list       emit invalid JSON instead of a session/list reply
 """
 import json
+import signal
 import sys
 import time
 
 
 MODES = set(sys.argv[1:])
 
+if "--ignore-term" in MODES:
+    signal.signal(signal.SIGTERM, signal.SIG_IGN)
+
 
 def send(payload):
     sys.stdout.write(json.dumps(payload) + "\n")