[BUG] stats command throw exception when alias of metrics with existing column name

[penghuo]

What is the bug?
stats command throw exception when alias of metrics with existing column name.

How can one reproduce the bug?

###
PUT {{baseUrl}}/log00001
Content-Type: application/x-ndjson

{
  "mappings": {
    "properties": {
      "@timestamp": {
        "type": "date"
      },
      "age": {
        "type": "integer"
      }
    }
  }
}

###
GET {{baseUrl}}/log00001

###
POST {{baseUrl}}/log00001/_bulk
Content-Type: application/x-ndjson

{ "index": { "_id": 1 } }
{"@timestamp":"2025-08-19T04:53:24Z", "age":10}
{ "index": { "_id": 2 } }
{"@timestamp":"2025-08-20T04:53:24Z", "age":11}

###
POST {{baseUrl}}/_plugins/_ppl/
Content-Type: application/x-ndjson

{
  "query": "source=log00001 | stats sum(age) as age by span(@timestamp, 1d)"      --> age is existing column name
}

{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "class java.lang.Integer cannot be cast to class java.lang.Long (java.lang.Integer and java.lang.Long are in module java.base of loader 'bootstrap')",
    "type": "ClassCastException"
  },
  "status": 500
}

What is the expected behavior?
A clear and concise description of what you expected to happen.

[qianheng-aws]

Addressed by #4500