[BUG] Cannot Use _source With Execute Painless Script API

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Describe the bug

According to documentation, with the filter context and supplying an in-memory doc:

The filter context runs the script as if the script were inside a script query. You must provide a test document in the context. The _source, stored fields, and _doc variables will be available to the script.

If I supply a "_source" attribute, OpenSearch complains about unknown field.

{
   "context_setup":
     "_source": 
...
}

{"error":{"root_cause":[{"type":"x_content_parse_exception","reason":"[1:239] [execute_script_context] unknown field [_source]"}],"type":"x_content_parse_exception","reason":"[1:250] [painless_execute_request] failed to parse field [context_setup]","caused_by":{"type":"x_content_parse_exception","reason":"[1:239] [execute_script_context] unknown field [_source]"}},"status":400}

If I include it in the document, I get metadata error:

{
   "context_setup":
    "document": {
      "_source": 
...
}

{"error":{"root_cause":[{"type":"mapper_parsing_exception","reason":"Field [_source] is a metadata field and cannot be added inside a document. Use the index API request parameters."}],"type":"mapper_parsing_exception","reason":"failed to parse field [_source] of type [_source] in document with id '_id'. Preview of field's value: '{function=ecc, name=sec-t571k1, type=implementation}'","caused_by":{"type":"mapper_parsing_exception","reason":"Field [_source] is a metadata field and cannot be added inside a document. Use the index API request parameters."}},"status":400}

It is not clear how an in-memory document as part of the Request can include a _source per the document. I am testing a script that is accessing _source.

Related component

Other

To Reproduce

Issue a REST call to /_scripts/painless/_execute with a document containing a _source attribute.

{
  "script": {
    "source": "doc['grad'].value == true && doc['gpa'].value >= params.min_honors_gpa",
    "params": {
      "min_honors_gpa": 3.5
    }
  },
  "context": "filter",
  "context_setup": {
    "index": "testindex1",
    "document": {
     "_source": { ... }
      "grad": true,
      "gpa": 3.79
    }
  }
}

Expected behavior

Script can access the _source attribute per the documentation.

The filter context runs the script as if the script were inside a script query. You must provide a test document in the context. The _source, stored fields, and _doc variables will be available to the script.

Additional Details

No response