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>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>
```

### Gradle Setup
```kotlin
dependencies {
  implementation("org.springframework.boot:spring-boot-starter-json")
  testImplementation("org.springframework.boot:spring-boot-starter-test")
}
```

### Basic Serialization and Deserialization

```java
@JsonTest
class UserDtoJsonTest {

  @Autowired
  private JacksonTester<UserDto> json;

  @Test
  void shouldSerializeUserToJson() throws Exception {
    UserDto user = new UserDto(1L, "Alice", "alice@example.com", 25);
    JsonContent<UserDto> result = json.write(user);

    result
      .extractingJsonPathNumberValue("$.id").isEqualTo(1)
      .extractingJsonPathStringValue("$.name").isEqualTo("Alice")
      .extractingJsonPathStringValue("$.email").isEqualTo("alice@example.com")
      .extractingJsonPathNumberValue("$.age").isEqualTo(25);
  }

  @Test
  void shouldDeserializeJsonToUser() throws Exception {
    String json_content = "{\"id\":1,\"name\":\"Alice\",\"email\":\"alice@example.com\",\"age\":25}";
    UserDto user = json.parse(json_content).getObject();

    assertThat(user.getId()).isEqualTo(1L);
    assertThat(user.getName()).isEqualTo("Alice");
    assertThat(user.getEmail()).isEqualTo("alice@example.com");
    assertThat(user.getAge()).isEqualTo(25);
  }

  @Test
  void shouldHandleNullFields() throws Exception {
    String json_content = "{\"id\":1,\"name\":null,\"email\":\"alice@example.com\"}";
    UserDto user = json.parse(json_content).getObject();
    assertThat(user.getName()).isNull();
  }
}
```

### Custom JSON Properties

```java
public class Order {
  @JsonProperty("order_id")
  private Long id;

  @JsonProperty("total_amount")
  private BigDecimal amount;

  @JsonIgnore
  private String internalNote;
}

@JsonTest
class OrderJsonTest {

  @Autowired
  private JacksonTester<Order> json;

  @Test
  void shouldMapJsonPropertyNames() throws Exception {
    String json_content = "{\"order_id\":123,\"total_amount\":99.99}";
    Order order = jso