[Bug] UpdateItem AttributeUpdates DELETE returns empty Attributes with UPDATED_NEW

[yesyayen]

Describe the bug

UpdateItem with AttributeUpdates that uses Action: DELETE on a Map attribute, with ReturnValues: UPDATED_NEW. The DELETE removes the attribute, and the UPDATED_NEW return mode asks for the new value of any attribute that was added or updated (not deleted). Real DynamoDB returns the response with no Attributes field at all. ExtendDB returns the response with an empty Attributes map (Attributes: {}).

To Reproduce

Items present:

[
  {
    "pk": {
      "S": "p"
    },
    "sk": {
      "S": "s"
    },
    "map_attr": {
      "M": {
        "child": {
          "S": "old"
        }
      }
    }
  }
]

Request (UpdateItem):

{
  "TableName": "test-table",
  "Key": {
    "pk": {
      "S": "p"
    },
    "sk": {
      "S": "s"
    }
  },
  "AttributeUpdates": {
    "map_attr": {
      "Action": "DELETE"
    }
  },
  "ReturnValues": "UPDATED_NEW"
}

Expected behavior

DynamoDB responds with HTTP 200:

{}

Actual behavior

ExtendDB responds with HTTP 200:

{
  "Attributes": {}
}

Environment

• ExtendDB version: 0.1.0
• Operating system: macOS 26
• Rust version (if building from source): 1.94
• Client SDK/driver and version: CLI
• Deployment method (binary, container, source): source

Logs / stack trace

(Not applicable: this divergence is detected by differential wire-level comparison against real Amazon DynamoDB; no ExtendDB log is required to reproduce.)

Additional context

Detected by an automated DynamoDB conformance harness. Classifications: BODY_SHAPE_MISMATCH

Checklist

• I have searched existing issues for duplicates
• I have included the ExtendDB version and environment details
• I can reproduce this with the latest release