[FEATURE]: Relax batch type to accept BatchItem<'sqlite'>[] in DrizzleD1Database

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

The current `batch` method in `DrizzleD1Database` has the following signature:

```ts
async batch, T extends Readonly<[U, ...U[]]>>(
batch: T,
): Promise>
```

While this enforces a non-empty array, it makes it cumbersome to use in practice. For example, consider a helper function like:

```ts
function insertData(data: TData): BatchItem<'sqlite'>[] {
// ...
}
```

To use this with `batch`, we must cast the result:

```ts
await db.batch(insertData(data) as [BatchItem<'sqlite'>, ...BatchItem<'sqlite'>[]]);
```

This cast is verbose and error-prone, especially when you're already returning a typed array.

### Suggested Change

Relax the type constraint on `batch` to accept any readonly array of `BatchItem<'sqlite'>`:

```ts
async batch, T extends readonly U[]>(
batch: T,
): Promise>
```

A simple runtime check can return early for an empty array.
If this change is acceptable, I’d be happy to open a PR to implement it.

Start at the DrizzleD1Database.batch signature described in the issue and inspect its surrounding implementation. Verify the empty-array behavior and relevant batch tests, then confirm that readonly BatchItem<'sqlite'>[] is accepted without a cast while existing batch response typing remains intact.

Written by the indexing model from the issue text.