#25387 [Java][Spring] useRecords: records with additionalProperties

Fifth step towards #10490. With useRecords and useSpringBoot4, a model with additionalProperties becomes a record whose last component is the map, under @JsonAnySetter and @JsonAnyGetter. Jackson reads such a component only from 2.18, and Spring Boot 3 brings 2.17, so on Boot 3 these models stay classes and the reason is logged. Unlike the class, the map keeps keys in reverse document order.

@cachescrubber @welshm @MelleD @atextor @manedev79 @javisst @borsch @banlevente @Zomzog @martin-mfg @KannaKim

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Summary by cubic

With useRecords on Spring Boot 4, a model with additionalProperties is now generated as a record whose map of undeclared properties is its last component, annotated with @JsonAnySetter and @JsonAnyGetter. On Spring Boot 3 the model stays a class and the reason is logged, because reading the map through a record constructor requires Jackson 2.18 and the generated Boot 3 project brings 2.17.

• The compact constructor gives an absent map an empty LinkedHashMap, and getAdditionalProperty(key) accesses entries.
• Builders accept and copy the map; toBuilder keeps the map's entries.
• Models bound from request parameters and models with a property named additionalProperties also stay classes.
• Adds the spring-boot-4-records sample and its tests.

A model generated this way keeps map keys in reverse document order, unlike the class.