Fix exists queries on nested flat_object fields throws exception

[kkewwei]

Description

[Describe what this change achieves]

Related Issues

Resolves #16795

Check List

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

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[github-actions]

❌ Gradle check result for 5763a44: 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 7d38155: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.01%. Comparing base (42dc22e) to head (7d38155).
Report is 47 commits behind head on main.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16803      +/-   ##
============================================
- Coverage     72.05%   72.01%   -0.04%     
+ Complexity    65183    65157      -26     
============================================
  Files          5318     5318              
  Lines        303993   303996       +3     
  Branches      43990    43992       +2     
============================================
- Hits         219028   218923     -105     
- Misses        67046    67111      +65     
- Partials      17919    17962      +43     

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[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.

[github-actions]

❌ Gradle check result for 25e6440: 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?