Validate training completion handoff

scripts/supervise_container_yolo_training.py

@@ -37,6 +37,13 @@ def write_state(path: Path, payload: dict) -> None:
     temporary.replace(path)
 
 
+def load_completion_command(path: Path) -> list[str]:
+    command = json.loads(path.read_text(encoding="utf-8"))
+    if not isinstance(command, list) or not command or not all(isinstance(x, str) and x for x in command):
+        raise ValueError("completion command must be a non-empty JSON list of non-empty strings")
+    return command
+
+
 def main() -> int:
     parser = argparse.ArgumentParser()
     parser.add_argument("--container", required=True)
@@ -64,9 +71,11 @@ def main() -> int:
                 write_state(state_path, state)
                 return 0
             if args.completion_command_json:
-                command = json.loads(args.completion_command_json.read_text(encoding="utf-8"))
-                if not isinstance(command, list) or not command or not all(isinstance(x, str) and x for x in command):
+                try:
+                    command = load_completion_command(args.completion_command_json)
+                except (OSError, json.JSONDecodeError, ValueError) as exc:
                     state["status"] = "invalid_completion_command"
+                    state["completion_handoff_error"] = str(exc)
                     write_state(state_path, state)
                     return 4
                 result = subprocess.run(command, check=False)