Mirror upstream elastic/elasticsearch#134851 for AI review (snapshot of HEAD tree)

[phananh1010]

_type isn't special in ingest conditional scripts anymore, and we shouldn't warn about it as if it is.

With this pipeline:

PUT _ingest/pipeline/underscore_type
{
  "processors": [
    {
      "script": {
        "source": "ctx._foo = ctx._bar"  # this is a field
      }
    },
    {
      "script": {
        "source": "ctx._baz = ctx._type" # no warning from this, it's not a _conditional_
      }
    },
    {
      "set": {
        "if": "ctx._type == null", # this warns on `main` but won't anymore after this PR
        "field": "message_1",
        "value": "test"
      }
    },
    {
      "set": {
        "if": "ctx._missing == null", # this is just another field
        "field": "message_2",
        "value": "some other test"
      }
    }
  ]
}

This _simulate call:

POST _ingest/pipeline/underscore_type/_simulate
{
  "docs": [
    {
      "_index": "index",
      "_id": "id",
      "_source": {
        "hello": "world",
        "_bar": "quux"
      }
    }
  ]
}

gives this result on main:

// Warning: 299 Elasticsearch-9.2.0-494dc7075a299cb5edb06c36681601550c4fcdae "[types removal] Looking up doc types [_type] in scripts is deprecated."
{
  "docs" : [
    {
      "doc" : {
        "_index" : "index",
        "_version" : "-3",
        "_id" : "id",
        "_source" : {
          "_baz" : null,
          "message_2" : "some other test",
          "message_1" : "test",
          "_bar" : "quux",
          "hello" : "world",
          "_foo" : "quux"
        },
        "_ingest" : {
          "timestamp" : "2025-09-16T21:36:01.143913Z"
        }
      }
    }
  ]
}

The result would be the same after this PR, there just wouldn't be a warning anymore -- that is, _type is just a regular old non-special field, like _foo or _bar or whatever (edit: that is, at least in terms of reads).

Related to elastic#134816, in that I'm pulling a part of that out as its own PR for searchability and accounting purposes.

[phananh1010]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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