test: reproduce nullable Java component references

[janeklb]

Motivation

Java @Nullable component-reference properties are emitted as non-nullable bare $ref schemas in OpenAPI 3.0. This draft adds a focused regression test for #3362.

Modification

• Add a WebMVC endpoint returning a response with a @Nullable Java component reference.
• Assert that generated OpenAPI 3.0 scopes nullability to the property through a nullable allOf reference wrapper.

Result

The new test currently fails because Springdoc generates a bare $ref without the expected nullable wrapper. It provides a minimal executable reproduction for the issue.