[Bug Fix] Fix the backward compatibility regression with COMPLEMENT for Regexp queries introduced in OpenSearch 3.0

[prudhvigodithi]

Description

Coming from @msfroh suggestion #18397 (comment) I can see the issue where the COMPLEMENT is dropped and sanitisedSyntaxFlag returns as 0. With this change I see the value as 65536 (0x10000) which works as expected.

Added an IT test as I assume we cannot test this with existing unit tests in RegexpQueryBuilderTests.

Background, the COMPLEMENT(RegExp.DEPRECATED_COMPLEMENT), is updated as part of #16366

Note

When we upgrade to Lucene 11 (OpenSearch 4.0), we should:

• Remove COMPLEMENT(RegExp.DEPRECATED_COMPLEMENT), from OpenSearch RegexpFlag class.
• Handle the RegexpQueryBuilder.java
• Update Documentation https://docs.opensearch.org/docs/latest/query-dsl/regex-syntax/ by removing all references to the ~ operator and remove COMPLEMENT from the flags list

CC: @getsaurabh02

Related Issues

#18397

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 cef249f: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.72%. Comparing base (0874e58) to head (b445a49).
Report is 9 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18640      +/-   ##
============================================
- Coverage     72.80%   72.72%   -0.09%     
+ Complexity    68437    68363      -74     
============================================
  Files          5563     5563              
  Lines        314174   314177       +3     
  Branches      45554    45555       +1     
============================================
- Hits         228726   228474     -252     
- Misses        66871    67166     +295     
+ Partials      18577    18537      -40     

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