fix(cron): validate expression syntax in _validate_schedule_for_add

nanobot/cron/service.py

@@ -75,10 +75,17 @@ def _validate_schedule_for_add(schedule: CronSchedule) -> None:
     if schedule.tz and schedule.kind != "cron":
         raise ValueError("tz can only be used with cron schedules")
 
-    if schedule.kind == "cron" and schedule.tz:
+    if schedule.kind == "cron":
+        if not schedule.expr or not schedule.expr.strip():
+            raise ValueError("cron schedule requires a non-empty 'expr'")
+        try:
+            from croniter import croniter
+            croniter(schedule.expr)
+        except Exception as e:
+            raise ValueError(f"invalid cron expression '{schedule.expr}': {e}") from None
+        if schedule.tz:
             try:
                 from zoneinfo import ZoneInfo
-
                 ZoneInfo(schedule.tz)
             except Exception:
                 raise ValueError(f"unknown timezone '{schedule.tz}'") from None