How do I query the cart line properties attribute?

FYI, There’s a better way to do this using Inline Fragments:

query RunInput {
  cart {
    lines {
      id
      quantity
      attributes {
        ... on Attribute {
          key
          value
        }
      }
    }
  }
}

That will give you an array of all the attributes:

"attributes": [
  {
    "key": "test1",
    "value": "true"
  },
  {
    "key": "test2",
    "value": "false"
  }
]