[BUG] update document with _bulk fails to generate embeddings in inference processors

[will-hwang]

Describe the bug

Bug Description:

embeddings are not generated when documents are updated with /index/_bulk operation, when successfully generated with insert /index/_doc/ operation. Is there any change made in Opensearch that skips embedding generation for bulk operation only?

How to reproduce the error:

Can you please assist with this issue?

Related component

No response

To Reproduce

1. deploy model

POST /_plugins/_ml/models/{model_id}/_deploy

2. configure pipeline

"processors": [
    {
      "text_embedding": {
        "model_id": {model_id},
        "field_map": {
          "text": "passage_embedding"
        }
      }
    }

3. ingest doc

PUT /my-nlp-index/_doc/1
{
  "text": "hello world"
}

4. update doc with _bulk

PUT /my-nlp-index/_bulk
{ "update": { "_index": "my-nlp-index", "_id": "1" } }
{ "doc" : { "text": "bye world" } }

Expected behavior

Embeddings are created for the initial ingest for "text":"hello world", but not updated with bulk operation
{ "doc" : { "text": "bye world" } }

Embeddings should be re-generated for bulk operation by calling text_embedding_processor

Additional Details

Plugins
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Screenshots
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Host/Environment (please complete the following information):

• OS: [e.g. iOS] MAC OS
• Version [e.g. 22] Sequoia 15.3

Additional context
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