Support ZodReadonly

a readonly schema results in the error
`Unable to generate valid values for Zod schema. 'ZodReadonly' schemas are not supported (at path '.').`

Based on what [zod does](https://zod.dev/?id=readonly) under the hood, do a map and return Object.freeze() on the value.

Start by locating the schema-to-arbitrary mapping for unsupported ZodReadonly schemas and compare it with Zod's readonly behavior described in the linked documentation. Verify that generated values are frozen and that the existing unsupported-schema error no longer occurs.

Written by the indexing model from the issue text.