test: terminate server process when finished (#2700)

# What does this PR do?
Terminate server process for real.

## Test Plan
```ENABLE_OPENAI=openai LLAMA_STACK_CONFIG=server:starter pytest -v tests/integration/agents/test_openai_responses.py --text-model "gpt-4o-mini" -vv -s -k 'test_list_response_input_items[' && lsof -ti:8321```
observe no process printed anymore

tests/integration/fixtures/common.py

@@ -6,6 +6,7 @@
 
 import inspect
 import os
+import signal
 import socket
 import subprocess
 import tempfile
@@ -45,6 +46,8 @@ def start_llama_stack_server(config_name: str) -> subprocess.Popen:
         stderr=subprocess.PIPE,  # keep stderr to see errors
         text=True,
         env={**os.environ, "LLAMA_STACK_LOG_FILE": "server.log"},
+        # Create new process group so we can kill all child processes
+        preexec_fn=os.setsid,
     )
     return process
 
@@ -267,14 +270,17 @@ def cleanup_server_process(request):
                 print(f"Server process already terminated with return code: {server_process.returncode}")
                 return
             try:
-                server_process.terminate()
+                print(f"Terminating process {server_process.pid} and its group...")
+                # Kill the entire process group
+                os.killpg(os.getpgid(server_process.pid), signal.SIGTERM)
                 server_process.wait(timeout=10)
-                print("Server process terminated gracefully")
+                print("Server process and children terminated gracefully")
             except subprocess.TimeoutExpired:
                 print("Server process did not terminate gracefully, killing it")
-                server_process.kill()
+                # Force kill the entire process group
+                os.killpg(os.getpgid(server_process.pid), signal.SIGKILL)
                 server_process.wait()
-                print("Server process killed")
+                print("Server process and children killed")
             except Exception as e:
                 print(f"Error during server cleanup: {e}")
         else: