Fix ResourceSupport not serializable on spring boot 2.1.x

[ruthst00]

Fixes #1726

Root cause: DomainObjectReader.readPut() passes the raw ObjectNode (including _links) to mapper.readerFor(target.getClass()).readValue(...) for intermediate deserialization. Jackson sees _links, finds the inherited links field on RepresentationModel, and tries to set it — but it's unmodifiable.

Fix (5 files changed, 173 insertions):

1. MappedJacksonProperties — Added isKnownJacksonProperty(String name) which returns true if a field is a mapped persistent property, an unmapped Jackson property (e.g. @Transient fields), or covered by @JsonAnySetter. Fields like _links that are unknown to Jackson return false.
2. DomainObjectReader.readPut() — Added stripNonWritableFields() that deep-copies the ObjectNode and removes any fields not known to Jackson before the intermediate deserialization step.
3. PersistentEntityResourceHandlerMethodArgumentResolver — Added the same stripping logic for the POST (create) path, plus a new PersistentEntities constructor parameter wired via RepositoryRestMvcConfiguration.
4. RepositoryRestMvcConfiguration — Passes entities to the new constructor.
5. DomainObjectReaderUnitTests — Added RepresentationModelEntity fixture and readPutWithLinksFieldDoesNotThrowForRepresentationModelSubclass test. All 43 tests pass.

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