Fix exists query for flat object

[bugmakerrrrrr]

Description

The current implementation of exists query for subfield uses an inverted index under <field name> to perform the query, this inverted index create mapping for each level of subfields. Taking a field field1 with flat_object type and a doc with value {"field1": {"field2" : {"field3": "a"}}}, the inverted index under field1 field contains terms field1.field2 and field1.field3, when we run the query {"query": {"exists": {"field": "field1.field2.field3"}}}, it turns into term query "field1": "field1.field2.field3", there is no match term obviously.

Since PathAndValue field contains <full path>=<value> term or doc values, we can use the user specified field name append = symbol as prefix to run the query, which equals to range query with both lowerTerm and upperTerm are null.

Related Issues

Resolves #17069

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 354123c: 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 98c36e0: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.30%. Comparing base (931c1aa) to head (98c36e0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #17108      +/-   ##
============================================
- Coverage     72.30%   72.30%   -0.01%     
+ Complexity    65482    65437      -45     
============================================
  Files          5309     5309              
  Lines        304324   304323       -1     
  Branches      44132    44132              
============================================
- Hits         220056   220038      -18     
+ Misses        66259    66222      -37     
- Partials      18009    18063      +54     

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[bugmakerrrrrr]

hi @msfroh , should we backport this to 2.x since it is a bug?