Unit Test Config Properties

Tests Spring Boot configuration properties without spinning up the full application context, which means your tests run fast. Uses ApplicationContextRunner to verify property binding, validation constraints, type conversions (like Duration and DataSize), and nested structures. The skill covers the usual pain points: making sure your kebab-case YAML properties actually map to camelCase fields, that @Validated annotations reject bad input, and that profile-specific configs work as expected. It's focused on the @ConfigurationProperties annotation specifically, so if you're testing how application.properties or YAML files bind to your config classes, this gives you the patterns. Includes validation testing with AssertJ assertions and edge case handling.