Provide a SlicedModel DTO to render stable JSON representations of Slice

[arnabnandy7]

Provide a SlicedModel DTO to render stable JSON representations of Slice.

org.springframework.data.web.PagedModel was introduced in 3.3 via #3024 as a middle ground between serializing PageImpl directly and adopting Spring HATEOAS. There was previously no equivalent for Slice.

Problem

Returning a Slice from a Spring MVC controller carried stability risks:

1. SliceImpl is a domain type and serializing it directly exposes internal structure that can change across releases.
2. In DIRECT mode, no warning was emitted for SliceImpl instances.
3. Applications not using Spring HATEOAS (SlicedResourcesAssembler) had to define custom DTOs per project to wrap Slice.

Solution

This PR introduces SlicedModel<T> in org.springframework.data.web, mirroring PagedModel in shape and usage:

• SlicedModel<T> and SliceMetadata:
  • Exposes content and page metadata (size, number, numberOfElements, hasNext).
  • Deliberately omits totalElements and totalPages as that information is unavailable on a Slice.
  • Implements equals(Object) and hashCode().
• Jackson Integration (Jackson 2 & 3):
  • In PageSerializationMode.VIA_DTO, registers a mix-in on SliceImpl.class with SliceModelConverter to wrap SliceImpl instances into SlicedModel.
  • In PageSerializationMode.DIRECT, emits a one-time warning log highlighting stability risks and suggesting SlicedModel or SlicedResourcesAssembler.
• Web Support & Javadoc:
  • Updated @EnableSpringDataWebSupport and PageSerializationMode Javadocs to document SliceImpl and SlicedModel.
• AOT / Native Hints:
  • Registered reflection hints in WebRuntimeHints for SlicedModel, SliceMetadata, and SliceModelConverter.
• Documentation & Tests:
  • Added Antora reference documentation for SlicedModel in core-extensions-web.adoc and aot.adoc.
  • Added unit and integration tests across SlicedModelUnitTests, PageImplJsonSerializationUnitTests, PageImplJsonJackson2SerializationUnitTests, EnableSpringDataWebSupportIntegrationTests, and WebRuntimeHintsUnitTests.

Closes #3516

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