unit-test-json-serialization

# Unit Testing JSON Serialization with `@JsonTest` ## Overview Provides patterns for unit testing JSON serialization and deserialization using Spring's `@JsonTest` and Jackson. Covers POJO mapping, custom serializers, field name mappings, nested objects, date/time formatting, and polymorphic types. ## When to Use - Testing JSON serialization/deserialization of DTOs - Verifying custom Jackson serializers/deserializers - Validating `@JsonProperty`, `@JsonIgnore`, and field name mappings - Testing date/time format handling (LocalDateTime, Date) - Testing null handling and missing fields - Testing polymorphic type deserialization ## Instructions 1. **Annotate test class with `@JsonTest`** → Enables JacksonTester auto-configuration 2. **Autowire JacksonTester for target type** → Provides type-safe JSON assertions 3. **Test serialization** → Call `json.write(object)` and assert JSON paths with `extractingJsonPath*` 4. **Test deserialization** → Call `json.parse(json)` or `json.parseObject(json)` and assert object state 5. **Validate round-trip** → Serialize, then deserialize, verify same data (if object is properly comparable) 6. **Test edge cases** → Null values, missing fields, empty collections, invalid JSON 7. **Add validation checkpoints**: After each assertion, verify the test fails meaningfully with wrong data ## Examples ### Maven Setup ```xml <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-json</artifactId> </dependency>...