Fix embedded ID relation response schemas

[hej090224]

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