fix(cron): accept list-form deliver values so deliver=['telegram'] works (#17456)

The cron schema contracts deliver as a string ("local", "origin",
"telegram", "telegram:chat_id[:thread_id]", or comma-separated combos),
but MCP clients and scripts sometimes pass an array like ['telegram'].

Before this change, the list was written to jobs.json verbatim, and
the scheduler's str(deliver).split(',') then tried to resolve the
literal string "['telegram']" as a platform — returning None and
logging 'no delivery target resolved for deliver=[\'telegram\']'.

Fix on both ends:
- tools/cronjob_tools.py: normalize deliver at the API boundary on
  create and update, so storage is always a string.
- cron/scheduler.py: normalize deliver in _resolve_delivery_targets,
  so existing jobs.json entries with list-form deliver are handled
  gracefully without requiring users to edit the file.

Closes #17139

cron/scheduler.py

@@ -233,12 +233,32 @@ def _resolve_single_delivery_target(job: dict, deliver_value: str) -> Optional[d
     }
 
 
+def _normalize_deliver_value(deliver) -> str:
+    """Normalize a stored/submitted ``deliver`` value to its canonical string form.
+
+    The contract is that ``deliver`` is a string (``"local"``, ``"origin"``,
+    ``"telegram"``, ``"telegram:-1001:17"``, or comma-separated combinations).
+    Historically some callers — MCP clients passing an array, direct edits of
+    ``jobs.json``, or stale code paths — have stored a list/tuple like
+    ``["telegram"]``.  ``str(["telegram"])`` would serialize to the literal
+    string ``"['telegram']"``, which is not a known platform and fails
+    resolution silently.  Flatten lists/tuples into a comma-separated string
+    so both forms work.  Returns ``"local"`` for anything falsy.
+    """
+    if deliver is None or deliver == "":
+        return "local"
+    if isinstance(deliver, (list, tuple)):
+        parts = [str(p).strip() for p in deliver if str(p).strip()]
+        return ",".join(parts) if parts else "local"
+    return str(deliver)
+
+
 def _resolve_delivery_targets(job: dict) -> List[dict]:
     """Resolve all concrete auto-delivery targets for a cron job (supports comma-separated deliver)."""
-    deliver = job.get("deliver", "local")
+    deliver = _normalize_deliver_value(job.get("deliver", "local"))
     if deliver == "local":
         return []
-    parts = [p.strip() for p in str(deliver).split(",") if p.strip()]
+    parts = [p.strip() for p in deliver.split(",") if p.strip()]
     seen = set()
     targets = []
     for part in parts: