Document `adminSecretsStore()` in `cloudflare:test` for vitest-pool-workers

## Summary

`@cloudflare/vitest-pool-workers` is adding a new `adminSecretsStore()` export to `cloudflare:test` (cloudflare/workers-sdk#13073). This gives tests write access to secrets store bindings, which are otherwise read-only (`.get()` only).

## Usage

```ts
import { adminSecretsStore } from "cloudflare:test";
import { env } from "cloudflare:workers";

const admin = adminSecretsStore(env.MY_SECRET);
await admin.create("test-value");

const value = await env.MY_SECRET.get(); // "test-value"
```

## Admin API methods

- `create(value: string): Promise` — create a secret, returns its ID
- `update(value: string, id: string): Promise` — update a secret by ID
- `duplicate(id: string, newName: string): Promise` — duplicate a secret under a new name
- `delete(id: string): Promise` — delete a secret by ID
- `list(): Promise<{ name: string; metadata?: { uuid: string } }[]>` — list all secrets
- `get(id: string): Promise` — get a secret's name by ID

## Where to document

This should be added to the [vitest-pool-workers testing documentation](https://developers.cloudflare.com/workers/testing/vitest-integration/), likely alongside the existing `applyD1Migrations()` documentation as another binding-seeding helper.