HDDS-16073. Implement ObjectParts listing and pagination for completed multipart objects in GetObjectAttributes

[Gargi-jais11]

What changes were proposed in this pull request?

Phase 1 only returns ObjectParts.TotalPartsCount from the ETag suffix.

• ObjectParts.Parts[] — Return PartNumber and Size for each part of a completed MPU object in the GetObjectAttributes XML response.
• Pagination — Honor x-amz-max-parts and x-amz-part-number-marker; populate MaxParts, PartNumberMarker, NextPartNumberMarker, and IsTruncated per the S3 API.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-16073

How was this patch tested?

Added unit test and integration test.
Manual testing:

// All parts (no pagination headers):
bash-5.1$ aws s3api get-object-attributes \
  --bucket attr-test \
  --key mpu-key \
  --object-attributes ObjectParts ETag ObjectSize \
  --endpoint-url $S3_ENDPOINT
{
    "LastModified": "Fri, 14 Aug 2026 05:27:28 GMT",
    "ETag": "c22cf5fdee60894e742ddda530c8b5b6-3",
    "ObjectParts": {
        "TotalPartsCount": 3,
        "IsTruncated": false,
        "Parts": [
            {
                "PartNumber": 1,
                "Size": 5242880
            },
            {
                "PartNumber": 2,
                "Size": 5242880
            },
            {
                "PartNumber": 3,
                "Size": 5242880
            }
        ]
    },
    "ObjectSize": 15728640
}

// Paginated — first page (max 2 parts):
bash-5.1$ aws s3api get-object-attributes \
  --bucket attr-test \
  --key mpu-key \
  --object-attributes ObjectParts \
  --max-parts 2 \
  --part-number-marker 0 \
  --endpoint-url $S3_ENDPOINT
{
    "LastModified": "Fri, 14 Aug 2026 05:27:28 GMT",
    "ObjectParts": {
        "TotalPartsCount": 3,
        "PartNumberMarker": 0,
        "NextPartNumberMarker": 2,
        "MaxParts": 2,
        "IsTruncated": true,
        "Parts": [
            {
                "PartNumber": 1,
                "Size": 5242880
            },
            {
                "PartNumber": 2,
                "Size": 5242880
            }
        ]
    }
}

// Second page (parts after marker 2):
bash-5.1$ aws s3api get-object-attributes \
  --bucket attr-test \
  --key mpu-key \
  --object-attributes ObjectParts \
  --max-parts 2 \
  --part-number-marker 2 \
  --endpoint-url $S3_ENDPOINT
{
    "LastModified": "Fri, 14 Aug 2026 05:27:28 GMT",
    "ObjectParts": {
        "TotalPartsCount": 3,
        "PartNumberMarker": 2,
        "MaxParts": 2,
        "IsTruncated": false,
        "Parts": [
            {
                "PartNumber": 3,
                "Size": 5242880
            }
        ]
    }
}