Upgrade Zod to v4

[sanjaysah101]

Description:

We should upgrade [zod](https://github.com/colinhacks/zod) from v3 to v4 to take advantage of the latest improvements, better DX, and upcoming ecosystem support.

Checklist:

• Upgrade zod to ^4.x
• Update all schemas and refinements to match new API (if any breaking changes)
• Update any custom error handling (e.g., .safeParse, .parse, or issue formatting)
• Run all tests and lint checks
• Update relevant documentation or comments

Reference:

• [Zod v4 Changelog](https://github.com/colinhacks/zod/releases)
• [Migration Guide](https://zod.dev/v4/changelog)

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Let me know if you'd like this written as a PR template or expanded into subtasks for a project board.