[Backport 2.x] Improve flat_object field parsing performance by reducing two passes to a single pass

[bugmakerrrrrr]

Description

backport #16297

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 cfc1aab: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 78.01418% with 31 lines in your changes missing coverage. Please review.

Project coverage is 71.93%. Comparing base (bf42c3e) to head (cfc1aab).
Report is 6 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	78.01%	13 Missing and 18 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #17126      +/-   ##
============================================
- Coverage     71.96%   71.93%   -0.04%     
- Complexity    65772    65792      +20     
============================================
  Files          5325     5324       -1     
  Lines        306459   306320     -139     
  Branches      44698    44685      -13     
============================================
- Hits         220554   220338     -216     
- Misses        67432    67626     +194     
+ Partials      18473    18356     -117     

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