BUG: Type Mismatch Between DurableObject Abstract Class and DurableObject Interface for fetch Method

## Abstract
I recently encountered a type mismatch issue while writing test code for Cloudflare Worker's DurableObject by `@cloudflare/vitest-pool-worker`. The problem arises due to the fetch method being undefined, following the example provided in [honojs/examples](https://github.com/honojs/examples/blob/main/durable-objects/src/counter.ts) for creating a DurableObject.

## Packages
- "@cloudflare/vitest-pool-workers": "0.2.6",
- "@cloudflare/workers-types": "4.20240502.0",
- "@faker-js/faker": "~8.4.1",
- "vitest": "1.3.0",
- "wrangler": "3.53.1"

## Codes
`src/Matchmaking.ts`
```typescript:src/Matchmaking.ts
export class MatchmakingDurableObject extends DurableObject {
async addQueue(firebaseUser: FirebaseUser): Promise{
// ...
}

async removeQueue(firebaseUser: FirebaseUser): Promise{
// ...
}
}
```
`test/Matchmaking.spec.ts`
```typescript:test/Matchmaking.spec.ts
describe("MatchmakingDurableObject", () => {
it("Successful addQueue", async () => {
const id = env.MATCHMAKING.idFromName("matchmaking");
const stub = env.MATCHMAKING.get(id);
await runInDurableObject(stub, async (instance, state) => {
instance.addQueue(MockFirebaseUsers[0]);
const stored = await state.storage.get("queue");
expect(stored).toBeTypeOf("string");
const queue = JSON.parse(stored as string)
expect(queue.length).toBe(1);
})
});
});
```
`tsconfig.json`
```json:tsconfig.json
{
"compilerOptions": {
"target": "ESNext",
"module": "ESNext",
"moduleResolution": "Bundler",
"strict": true,
"skipLibCheck": true,
"lib": [
"ESNext"
],
"types": [
"@cloudflare/workers-types/experimental",
],
"jsx": "react-jsx",
"jsxImportSource": "hono/jsx"
},
}
```
`test/tsconfig.json`
```json:test/tsconfig.json
{
"extends": "../tsconfig.json",
"compilerOptions": {
"moduleResolution": "bundler",
"types": [
"@cloudflare/workers-types/experimental",
"@cloudflare/vitest-pool-workers"
]
},
"include": ["./**/*.ts", "../src/env.d.ts"]
}
```
## Error log on vscode
```
Type 'MatchmakingDurableObject' does not satisfy the constraint 'DurableObject'.
Types of property 'fetch' are incompatible.
Type '((request: Request>) => Response | Promise) | undefined' is not assignable to type '(request: Request>) => Response | Promise'.
Type 'undefined' is not assignable to type '(request: Request>) => Response | Promise'.ts(2344)
```

## Notes
This commit of the example is just 1 week ago, so I know this is the new error.
https://github.com/honojs/examples/commit/9b7acf119e874f389ca386fb2406c8f2110039e6
Temporarily I just define the fetch method on the DurableObject for the test, strangely.