vite-plugin: `vite dev` does not throw an error when async i/o runs outside a request context

### What versions & operating system are you using?

`wrangler@4.19.1`, `@cloudflare/vite-plugin:1.5.0`

### Please provide a link to a minimal reproduction

https://github.com/threepointone/test-vite-wrangler-async-io

### Describe the Bug

workerd should throw an error when async i/o is run outside of a request context. for example, this script should throw:

```js
setTimeout(() => {
console.log("timer fired");
}, 1000);

export default {
async fetch(request, env, ctx) {
return new Response("Hello World");
},
};
```

when run with `wrangler dev`, this throws as expected. but it does NOT throw when run with `vite dev`.

### Please provide any relevant error logs

_No response_

Begin with the linked minimal reproduction and compare its behavior under `vite dev` and `wrangler dev`; then trace the `@cloudflare/vite-plugin` entry point used by `vite dev`. Done when the timer example raises the same async-I/O-outside-request-context error under `vite dev` as it does under `wrangler dev`, with regression coverage if the relevant test location is found.

Written by the indexing model from the issue text.