fix full_response false and no output mapping exceptions

[mingshl]

Description

When configuring ml inference processor with full_response_path is false with no output mapping throw exceptions "reason": "An unexpected error occurred: model inference output cannot find field name: $.inference_results"

When ml inference search response processor, configuring no output mapping, then the processors will add the default output mapping as following that will add a field inference_result to the documents or search hits, reading from the json path $.inference_result in the prediction results.

{
  "output_map": {
    "inference_result": "$.inference_result"
  }
}

But checking in the ml inference ingest processor, when there is no output mapping, the default output mapping should be

{
  "output_map": {
    "inference_result": null
  }
}

to keep it consistent on ingest and search side, should have the same behaviour for the same mappings.

What is the expected behavior?
when inference processors has no output mapping,

when full_response_path is true, should default add the the entire response from the predict API

{
  "inference_results": [
    {
      "output": [
        {
          "name": "response",
          "dataAsMap": {
            "response": """ Based on the given context of ["hello world","hi earth","howdy there"], here is a summary of the documents: All three documents appear to be simple greetings using different words."""
          }
        }
      ],
      "status_code": 200
    }
  ]
}

when full_response_path is false, should add the prediction results within the tensors

 {
            "response": """ Based on the given context of ["hello world","hi earth","howdy there"], here is a summary of the documents: All three documents appear to be simple greetings using different words."""
          }

Related Issues

[Resolves #[Issue number to be closed when this PR is merged]

](https://github.com//issues/2943)

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[ohltyler]

LGTM, thanks for the quick fix!

[opensearch-trigger-bot]

The backport to 2.16 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.16 2.16
# Navigate to the new working tree
cd .worktrees/backport-2.16
# Create a new branch
git switch --create backport/backport-2944-to-2.16
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 853efd6be34f981740dec99dd649754eb085de83
# Push it to GitHub
git push --set-upstream origin backport/backport-2944-to-2.16
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.16

Then, create a pull request where the base branch is 2.16 and the compare/head branch is backport/backport-2944-to-2.16.