docs: update changelog with test reorganization

Add details about test file reorganization and coverage improvements
from 63.87% to 84.84% in the unreleased section.

CHANGELOG.md

@@ -23,7 +23,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Centralize TEMPLATES for consistent string formatting
 - Replace custom HTTP_STATUS dict with Python's standard `http.HTTPStatus` enum
 - Update AbstractProcessor and MetadataProcessor to use centralized constants
-- Improve test coverage from 61.71% to 63.87%
+- Reorganize and consolidate test files for better organization and clarity
+- Rename test files to better reflect their current purpose:
+  - `test_doi2dataset.py` → `test_validation_utils.py`
+  - `test_fetch_doi_mock.py` → `test_integration.py`
+  - `test_person.py` → `test_models.py`
+- Consolidate overlapping test concerns into dedicated files
+- Extract CLI tests into dedicated `test_cli.py` module
+- Improve test coverage from 63.87% to 84.84%
 
 ### Fixed