Description

We should be able to detect when a user is attempting to use copy_to to copy to a non-existent field when dynamic mappings are disabled. For example:

PUT test-index
{
  "mappings": {
    "dynamic": false,
    "properties": {
      "test_field": {
        "type": "text",
        "copy_to": "missing_field"
      }
    }
  }
}

Currently, this request succeeds even though nothing will ever be copied to missing_field. We should throw a 400 error in this case to indicate to the user that their mappings are invalid.