Doc pages breaking with Zod's bigint type

Using the `bigint` type of Zod seems to be breaking the `/docs` and `redocs` pages. A simple example based on the user guide, modified to use Zod's `bigint` instead of the normal `Int`:

```ts
import { OpenAPIRouter, OpenAPIRoute, Path, Int } from '@cloudflare/itty-router-openapi';
import { z } from 'zod'

export class ToDoFetch extends OpenAPIRoute {
static schema = {
parameters: {
todoId: Path(z.coerce.bigint().nonnegative(), { // <-- using `bigint` here
description: 'ToDo ID',
}),
},
}

async handle(
request: Request,
env: any,
context: any,
data: any
) {
const { todoId } = data.params
return new Response(todoId);
}
}

const router = OpenAPIRouter()
router.get('/todos/:todoId', ToDoFetch)

export default {
fetch: router.handle,
}
```

The `docs` page then throws: "Failed to load API definition. Fetch error response status is 500 /openapi.json"
The `redocs` page is stuck in "Loading..."

When using the standard `Int` type instead, the doc pages load normally.

Reproduce the supplied TypeScript example with z.coerce.bigint() and inspect the /openapi.json response used by the /docs and /redocs pages. Compare it with the Int version; done means bigint schemas produce a valid API definition and both documentation pages load normally.

Written by the indexing model from the issue text.