fix: add explicit types to cron schedule schema for Moonshot

[caoyacheng]

Summary

• Moonshot/Kimi validates tool JSON Schema more strictly and rejects enum/const without type.
• Add explicit type: "string" to cron schedule schema fields so Kimi can call cron_create.

Test plan

• Use a Moonshot/Kimi model and create a cron/delay task via agent tools
• Confirm the previous schema error no longer appears