fix(java-restclient): Spring 7 wrong serializers setup

[alex-nt]

For spring-boot:4/spring:7 the appropriate way of adding the message converters in our context seems to be via addCustomConverter as those are always added.

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Summary by cubic

Fixes serialization in Spring Boot 4 RestClient clients by adding Jackson JSON/XML message converters via addCustomConverter instead of withJsonConverter/withXmlConverter. Ensures converters are always registered under Spring 7 / Boot 4, restoring correct request/response handling.

^{Written for commit 1c9e21b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files