[Tech debt] Improve test coverage - phase 1, meet minimal required level

[martin-gaievski]

Improve test coverage for repo, all packages should be above 80%.

Currently report is showing following packages that are below 80%:

• https://github.com/opensearch-project/neural-search/tree/main/src/main/java/org/opensearch/neuralsearch/plugin - 71%
• https://github.com/opensearch-project/neural-search/tree/main/src/main/java/org/opensearch/neuralsearch/query - 77%
• https://github.com/opensearch-project/neural-search/tree/main/src/main/java/org/opensearch/neuralsearch/search - 77%

Steps to repro:

1. Check above links for code coverage report. Click on every class and check which lines or methods are not covered, write tests for those lines.
2. If you write unit test and want to run it, do

   ./gradlew test

   that will execute changes. If you want to check how coverage changes after adding your test, you need to create a new PR with your changes, mark it as "Draft" and let CI run. Part of the CI execution is generation of new coverage report with your changes.

Some tips:

• check existing unit tests, e.g. https://github.com/opensearch-project/neural-search/blob/main/src/test/java/org/opensearch/neuralsearch/query/HybridQueryBuilderTests.java
• make sure all tests are passing on your local
• make sure tests are deterministic and stable, avoid hammering failing tests until it passes

[dbwiddis]

It would probably be helpful to post the coverage report link in the issue description :)