Avoid infinite loop in flat_object parsing

[msfroh]

Description

We had logic in flat_object parsing that would:

1. Try parsing a flat object field that is not an object or null.
2. Would see an END_ARRAY token, ignore it, and not advance the parser.

Combined, this would create a scenario where passing an array of strings for a flat_object would parse the string values, then loop infinitely on the END_ARRAY token.

Related Issues

Resolves #15982

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[msfroh]

@kkewwei -- would you mind taking a look at this change?

[github-actions]

❌ Gradle check result for 73e7e84: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 4ee50a1: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for dff498d: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 6ddd18b: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.90%. Comparing base (3937ccb) to head (6ddd18b).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	70.58%	4 Missing and 1 partial ⚠️
...ch/common/xcontent/JsonToStringXContentParser.java	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #15985      +/-   ##
============================================
- Coverage     71.90%   71.90%   -0.01%     
+ Complexity    64392    64290     -102     
============================================
  Files          5278     5280       +2     
  Lines        300877   300864      -13     
  Branches      43478    43473       -5     
============================================
- Hits         216351   216337      -14     
+ Misses        66747    66736      -11     
- Partials      17779    17791      +12     

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-15985-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 05dab3b7eb54a361af3583a322f0a748d6412836
# Push it to GitHub
git push --set-upstream origin backport/backport-15985-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-15985-to-2.x.

[msfroh]

Manual backport PR: #16026