Checksum algorithm not supported with CompleteMultipartUpload to S3

**Describe the bug**

When doing a MPU to S3 with checksum algorithm sha256, got the error
```
BadDigestThe sha256 you specified did not match the calculated checksum.
```
**To Reproduce**

```
use object_store::ObjectStoreExt;
use object_store::aws::AmazonS3Builder;
use object_store::path::Path;
use tracing_subscriber::EnvFilter;

#[tokio::main]
async fn main() {
let client = AmazonS3Builder::from_env()
.with_bucket_name("bucket-name")
.with_region("us-east-1")
.with_checksum_algorithm(object_store::aws::Checksum::SHA256)
.build()
.unwrap();

let mut upload = client.put_multipart(&Path::from("test-mpu")).await.unwrap();
let res = upload.complete().await.unwrap();
println!("Upload complete: {:?}", res);
}

```

**Expected behavior**

No error.

**Additional context**

Start with the provided Rust reproduction using AmazonS3Builder, with_checksum_algorithm(Checksum::SHA256), put_multipart, and upload.complete(). Trace the multipart completion request and checksum handling, then verify that the reproduction completes without S3 returning BadDigest.

Written by the indexing model from the issue text.