Fix issue where links are wrongly serialized when MappingJacksonValue is used as a return type

[ruthst00]

Fixes #1544

Root cause: In Spring Framework 7, MappingJacksonValue is deprecated and marked for removal. The Jackson 3 (tools.jackson) message converters (AbstractJacksonHttpMessageConverter) explicitly throw UnsupportedOperationException when they receive a MappingJacksonValue body, and instead use a hints map (Map<String, Object>) keyed by class name. When a @RepositoryRestController method returns a MappingJacksonValue wrapping a RepresentationModel, the HAL converter never sees the inner model — causing _links to be absent from the response.

Fix: Introduced MappingJacksonValueResponseBodyAdvice — a ResponseBodyAdvice that:

1. beforeBodyWrite() — detects MappingJacksonValue return values, unwraps the inner value (the RepresentationModel), and stashes any serializationView or FilterProvider hints in a ThreadLocal
2. determineWriteHints() — a new Spring 7 ResponseBodyAdvice hook that returns the stashed hints as a Map<String, Object> for the Jackson 3 converter to consume, then clears the ThreadLocal

The advice is registered first in RepositoryRestMvcConfiguration.repositoryExporterHandlerAdapter() so it runs before HalFormsAdaptingResponseBodyAdvice. A warning is logged when a Jackson 2 FilterProvider is detected (it cannot be bridged to Jackson 3's incompatible tools.jackson.databind.ser.FilterProvider API). All 8 unit tests pass.

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