Upgrade to Zod 4 dependency

[nova4lb]

It would be great if you can upgrade the usage of Zod to version 4 instead of 3.

It has native JSON Schema conversions which will make it easier to call tools with the openai package. It's currently not possible to install zod4 alongside openai, due to the zod version in openai's package.json.

[seratch]

Hi, thanks for sharing this feedback. Yes, we'll definitely migrate to zod 4 in the future, but it may take some time for several reasons. Please refer to #376 (comment) for more details.