[BUG] `model_id` on `SearchHitsHit` in ML schema is incorrect — field never exists in API response

[wanglam]

Bug Report

Description

The SearchHitsHit schema in spec/schemas/ml._common.yaml declares a model_id property:

SearchHitsHit:
  type: object
  properties:
    ...
    model_id:
      $ref: '_common.yaml#/components/schemas/Name'
    ...

However, the OpenSearch ML search APIs (_plugins/_ml/models/_search, _plugins/_ml/agents/_search, etc.) never return model_id as a top-level field on search hits. These APIs use standard OpenSearch SearchResponse — the document ID (which IS the model ID) is always in the standard _id field.

Reproduction

# Search for ML models
curl -s http://localhost:9200/_plugins/_ml/models/_search \
  -H "Content-Type: application/json" \
  -d '{"query":{"match_all":{}},"size":1}'

Response hit structure:

{
  "_index": ".plugins-ml-model",
  "_id": "6Wd_7p4BTcJJvPo5kauK",   // <-- this IS the model_id
  "_version": 1,
  "_seq_no": 0,
  "_primary_term": 1,
  "_score": 1.0,
  "_source": { ... }
}

Note: no model_id field at the hit level. The _id field contains the model ID.

Impact

This causes downstream clients (e.g., opensearch-js#1032) to generate incorrect TypeScript types where hit.model_id appears valid but is always undefined at runtime.

Root Cause

The SearchModelTransportAction in ML Commons simply delegates to OpenSearch's standard search handler (MLSearchHandler.search()), which returns a standard SearchResponse. No custom field injection occurs.

The model_id field correctly exists on the Source schema (inside _source for model chunks), but it should not be on SearchHitsHit.

Suggested Fix

Remove model_id from the SearchHitsHit schema in spec/schemas/ml._common.yaml (around line 75):

 SearchHitsHit:
   type: object
   properties:
     _version:
       $ref: '_common.yaml#/components/schemas/VersionNumber'
     _seq_no:
       $ref: '_common.yaml#/components/schemas/SequenceNumber'
     _primary_term:
       type: integer
     _index:
       $ref: '_common.yaml#/components/schemas/IndexName'
     _id:
       $ref: '_common.yaml#/components/schemas/Id'
     _score:
       type: ['null', number]
     _source:
       $ref: '#/components/schemas/Source'
-    model_id:
-      $ref: '_common.yaml#/components/schemas/Name'
     sort:
       type: array
       items:
         type: number
         format: float
   required:
     - _score

Verified On

• OpenSearch 3.6.0 (official tarball)
• OpenSearch 2.19 (reported by issue author in opensearch-js#1032)

The field has never been returned by the ML search API — this is not a regression but an incorrect spec definition since its introduction.