Refactor `PagedModel<T>`: Remove `Page<T>` properties and improve JSON serialization stability

[cmsong111]

Currently, PagedModel<T> directly holds a Page<T> instance, which introduces unnecessary complexity during JSON serialization and deserialization.

Therefore, it is suggested to remove the Page<T> attribute and leave only content and pageMetaData

Proposed Solution

• Remove the direct dependency on Page and instead manage only the necessary data.
• Simplify the internal structure by keeping only List content and PageMetadata page.
• Ensure proper deserialization support using a @JsonCreator constructor.

// Partial implementation highlighting the proposed changes
public class PagedModel<T> {

    private final List<T> content;
    private final PageMetadata page;

    public PagedModel(Page<T> page) {
        Assert.notNull(page, "Page must not be null");
        this.page = new PageMetadata(page.getSize(), page.getNumber(),
                page.getTotalElements(), page.getTotalPages());
        this.content = page.getContent();
    }

    @JsonCreator
    private PagedModel(
            @JsonProperty("content") List<T> content,
            @JsonProperty("page") PageMetadata metadata) {
        this.content = content;
        this.page = metadata;
    }
}

Improved Usability in Tests

By applying this change, writing test cases for API responses becomes more convenient, especially when using WebTestClient in a Spring Boot environment.

val result = webTestClient
    .get()
    .uri("/api/v1/test")
    .exchange()
    .expectStatus().isOk
    .expectBody(object : ParameterizedTypeReference<PagedModel<TestData>>() {})
    .returnResult()

result.responseBody.shouldNotBeNull()
result.responseBody.data?.content?.size shouldBe 5