[Feature]: Update Zod to ^4.0.0

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

Zod 4 was released almost a year ago. The libs in the AI-space have mostly moved onto it. AG-UI is on ^3.x.x which makes it impractical/difficult to use a lib that depends on Zod ^4 with AG-UI. Unfortunately, most libs don't list Zod as a peer-dependency like they should, nor import from `zod/v3` or `zod/v4` like they should.

### Proposed Solution

If AG-UI would just switch to Zod ^4, it would be mostly a non-issue.

### Alternatives Considered

_No response_

### Additional Context

_No response_