Fix embedded ID relation response schemas

[exijn]

Summary

• Convert nested response references for non-exported Spring Data REST associations to response schemas.
• Preserve embedded ID scalar properties while preventing association objects from being expanded in read response schemas.
• Add a minimal @EmbeddedId/@MapsId regression fixture and compare the complete generated OpenAPI 3.1 document with a golden file.

Root cause

SpringDocDataRestUtils rewrote top-level and _embedded entity response references, but did not rewrite associations to entities that were not themselves exported as repositories. Those properties retained the raw ModelConverters schema, which expanded the @EmbeddedId and @MapsId association fields recursively.

Compatibility

The change is limited to non-exported Spring Data REST association properties in generated response schemas. Existing exported relation handling, request schemas, and relation-free embeddables remain unchanged.

Testing

• mvn -pl springdoc-openapi-tests/springdoc-openapi-data-rest-tests -am -Dtest=SpringDocApp40Test '-Dsurefire.failIfNoSpecifiedTests=false' test
• mvn -pl springdoc-openapi-tests/springdoc-openapi-data-rest-tests -Dtest=SpringDocApp40Test '-Dsurefire.failIfNoSpecifiedTests=false' test
• mvn -pl springdoc-openapi-tests/springdoc-openapi-data-rest-tests test (79 tests pass)
• mvn install (the reactor reached springdoc-openapi-javadoc-tests, where existing Javadoc golden-output tests fail because descriptions, summaries, and tags are missing; the targeted app40 test and data REST module pass)

Fixes #3136

[bnasslahsen]

Merged, thank you.

I added two things on top of your change:

• v30/app40. Your regression test covers OAS 3.1 only, and the two versions take different branches when the _embedded items schema is resolved — JsonSchema for 3.1, ArraySchema for 3.0 — so the 3.0 path was untested. Reverting the fix makes the new test fail on components.schemas.Goal.properties.impactsByInitiatives.items.$ref and on the same property of EntityModelGoal, which is exactly the recursive expansion described in Incorrect schema/example with @EmbeddedId + relations (Spring Data REST) #3136.
• allAssociationsFieldsMap.getOrDefault(className, …) in updateResponseSchema, so an entity that reaches it without having been registered in the map cannot raise a NullPointerException.

Backported to spring-boot-3 in 998a254, adapted to that branch's Set<String> signatures and with its own golden files (spring-hateoas emits a different Link schema there).