feat(disk-storage): support opt-in flush for durable uploads (#1381)

[SAY-5]

Closes #1381.

Why

As reported in #1381, the disk storage pipes the uploaded file into a vanilla `fs.createWriteStream` and reports the upload as successful the moment the stream finishes. On Linux (and any OS with a write-back page cache) the bytes can still be sitting in kernel buffers when the request handler returns — a power loss or crash at the wrong moment leaves the file truncated or zero-length, but the HTTP response already claimed success.

Node.js 20.10 / 21.0 added a `flush` option to `fs.createWriteStream` that calls `fdatasync()` on the file descriptor before the stream closes, which is exactly what this workflow needs.

Change

Expose `flush` as an opt-in `DiskStorage` option:

```js
multer.diskStorage({
destination: './uploads',
flush: true
})
```

• Default stays `false`, so existing users get the exact same buffered-write behavior and the same throughput — no perf regression.
• When `flush` is `true`, the option is forwarded as `fs.createWriteStream(path, { flush: true })`. On runtimes older than Node 20.10, `createWriteStream` silently ignores the option, which matches the documented upstream behavior.
• No change to `_removeFile` or error handling.

Tests

Added two regression tests in `test/disk-storage.js`:

1. With `flush: true`, the test wraps `fs.createWriteStream` and asserts it was called with `{ flush: true }` while the upload still round-trips at the expected size.
2. Without `flush`, `createWriteStream` is still called with no options — verifies the default behavior is unchanged.

Full `npm test` suite: 74/74 passing.