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

tests/cron/test_scheduler.py

@@ -279,6 +279,44 @@ class TestResolveDeliveryTarget:
             "thread_id": None,
         }
 
+    def test_list_form_deliver_is_normalized(self, monkeypatch):
+        """deliver=['telegram'] (Python list) should resolve like 'telegram' string.
+
+        Regression test for #17139: MCP clients / scripts that pass the deliver
+        field as an array-shaped value used to fail with "no delivery target
+        resolved for deliver=['telegram']" because ``str(['telegram'])`` was
+        passed through to ``split(',')`` verbatim.
+        """
+        monkeypatch.setenv("TELEGRAM_HOME_CHANNEL", "-4004")
+        job = {
+            "deliver": ["telegram"],
+            "origin": None,
+        }
+
+        assert _resolve_delivery_target(job) == {
+            "platform": "telegram",
+            "chat_id": "-4004",
+            "thread_id": None,
+        }
+
+    def test_list_form_multiple_platforms_normalized(self, monkeypatch):
+        """deliver=['telegram', 'discord'] resolves to multiple targets."""
+        from cron.scheduler import _resolve_delivery_targets
+
+        monkeypatch.setenv("TELEGRAM_HOME_CHANNEL", "-111")
+        monkeypatch.setenv("DISCORD_HOME_CHANNEL", "-222")
+        job = {"deliver": ["telegram", "discord"], "origin": None}
+
+        targets = _resolve_delivery_targets(job)
+        platforms = sorted(t["platform"] for t in targets)
+        assert platforms == ["discord", "telegram"]
+
+    def test_empty_list_form_deliver_resolves_to_local(self):
+        """deliver=[] is treated as local (no delivery)."""
+        from cron.scheduler import _resolve_delivery_targets
+
+        assert _resolve_delivery_targets({"deliver": []}) == []
+
 
 class TestDeliverResultWrapping:
     """Verify that cron deliveries are wrapped with header/footer and no longer mirrored."""