Add test coverage improvement plan document

[dillonvargo]

Description of what I changed

Added TEST_COVERAGE_IMPROVEMENT_PLAN.md — a documentation-only file that outlines a structured, phased approach to improving test coverage across openmrs-core. The document covers:

• Current coverage infrastructure (JaCoCo, Coveralls, SonarCloud) and test profiles
• A service-level and DAO-level audit identifying classes with no dedicated test files (e.g., ConditionService, DiagnosisService, SerializationService, and 9 Hibernate DAO classes)
• Action items for the known AOP coverage gap (TRUNK-5151) where BaseVoidHandler masks coverage of void*/retire* methods
• Recommendations for expanding integration tests (module lifecycle, database migrations, cross-service workflows)
• Instructions for leveraging existing tooling (JaCoCo reports, SonarCloud, Coveralls)
• A 6-phase prioritized implementation plan with timeline estimates
• Concrete code examples following project conventions (JUnit 5, naming rules, Mockito patterns)

No functional code changes — this is documentation only.

Issue I worked on

see https://issues.openmrs.org/browse/TRUNK-5151 (partially — the AOP coverage gap is one of several items addressed in this plan)

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.
• I have added tests to cover my changes. (If you refactored existing code that was well tested you do not have to add tests)
  • N/A — documentation only, no code changes
• I ran mvn clean package right before creating this pull request and added all formatting changes to my commit.
  • N/A — no Java changes
• All new and existing tests passed.
• My pull request is based on the latest changes of the master branch.

Items for Reviewer Attention

1. Verify gap analysis accuracy: The tables in sections 2.4 claim ConditionService, DiagnosisService, and SerializationService have no test files, and that 9 DAO classes are untested. These were determined by directory listing — please confirm no tests exist under alternate names or locations.
2. AOP bypass recommendation (section 3.1): The suggested approach of using @InjectMocks on PersonServiceImpl to bypass AOP should be validated — the void logic may live entirely in the AOP handler rather than the method body, which would change the testing strategy.
3. Line number references will drift: The document references specific pom.xml and source file line numbers that will become stale over time. Consider whether these references add enough value to justify maintenance.
4. Prioritization: Phase ordering and P0/P1/P2 assignments are subjective — review whether these align with the project's actual priorities.

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Link to Devin run: https://app.devin.ai/sessions/daeb89e4ebe64b078ac5575befeb4e41
Requested by: Dillon Vargo

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