Clean up unused validateFieldName() and use existing methods for TextEmbeddingProcessorIT

[yizheliu-amazon]

Description

Clean up unused validateFieldName() and use existing methods getHitCount() and getFirstInnerHit() for TextEmbeddingProcessorIT to simplify test code and fix build for backport 2.x

Related Issues

Using existing methods getHitCount() and getFirstInnerHit() may help to fix build error in #1071: https://github.com/opensearch-project/neural-search/actions/runs/12664516471/job/35292787707?pr=1071

/__w/neural-search/neural-search/src/test/java/org/opensearch/neuralsearch/processor/TextEmbeddingProcessorIT.java:297: error: cannot find symbol
            Map<String, Object> innerHitDetails = listOfHits.getFirst();
                                                            ^
  symbol:   method getFirst()
  location: variable listOfHits of type List<Map<String,Object>>

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.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[yizheliu-amazon]

CHANGELOG can be skipped

[heemin32]

LGTM. Thanks!

[opensearch-trigger-bot]

The backport to 2.x 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.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-1074-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 fea0a7f0ababf20e9eb421e311431cb49c10cfe5
# Push it to GitHub
git push --set-upstream origin backport/backport-1074-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1074-to-2.x.