refactor: migrate from zod-to-json-schema to zod-openapi for OpenAPI schema generation

- Replaced zod-to-json-schema with zod-openapi in user routes for improved schema handling.
- Updated response schemas to use createSchema from zod-openapi.
- Enhanced error handling in user routes to return consistent JSON responses.
- Cleaned up user data responses to ensure only primitive types are returned.
- Added global error handler in server setup to manage validation and internal errors uniformly.

Author: Lasim