fix: Update ProcessorGenerationContext test calls for new param

[RexyZhang2001]

Description

OpenSearch core added a parentAction parameter to the ProcessorGenerationContext record (opensearch-project/OpenSearch#22195), changing its constructor from (SearchRequest) to (SearchRequest, String). This broke compileTestJava for SemanticHighlightingFactoryTests, causing the neural-search integration test suite to fail to build.

This PR updates all 11 ProcessorGenerationContext constructor calls in the test file to pass the new parameter as null, since these tests don't exercise parent-action-specific behavior. No production code changes were needed since SemanticHighlightingFactory only reads context.searchRequest().

Related Issues

Resolves #1904

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[github-actions]

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🧪 PR contains tests

🔒 No security concerns identified

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.44%. Comparing base (094d089) to head (b642a7f).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1909      +/-   ##
============================================
- Coverage     83.45%   83.44%   -0.01%     
- Complexity     3898     3899       +1     
============================================
  Files           291      291              
  Lines         13844    13844              
  Branches       2304     2304              
============================================
- Hits          11553    11552       -1     
- Misses         1456     1458       +2     
+ Partials        835      834       -1     

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