[Backport 2.x] Flat object field should delegate to keyword field for most query types

[kkewwei]

Description

Backport 9ddee61 from #14383

Related Issues

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

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 4acf291: 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 4acf291: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 79.31034% with 18 lines in your changes missing coverage. Please review.

Project coverage is 71.73%. Comparing base (5947002) to head (4acf291).
Report is 17 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	78.68%	4 Missing and 9 partials ⚠️
...rg/opensearch/index/mapper/KeywordFieldMapper.java	80.76%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #16326      +/-   ##
============================================
- Coverage     71.74%   71.73%   -0.01%     
- Complexity    65022    65029       +7     
============================================
  Files          5302     5303       +1     
  Lines        303866   303843      -23     
  Branches      44192    44183       -9     
============================================
- Hits         218012   217970      -42     
- Misses        67648    67699      +51     
+ Partials      18206    18174      -32     

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[kkewwei]

@msfroh, please have a look when you are free.