fix: [BUG][JAVA] vendorExtension.x-class-extra-annotation doesn't generate lists of annotations correctly

[johnyherangi]

Fixes #23749

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Summary by cubic

Fixes Java/Spring codegen so x-class-extra-annotation accepts a single string or a YAML list and emits all annotations. Also defaults to an empty list when missing, and applies to allOf models and oneOf interfaces.

• Bug Fixes

  • Normalized x-class-extra-annotation to List<String> in AbstractJavaCodegen with an empty-list default.
  • Updated POJO and oneof_interface templates to loop and emit each annotation using {{{.}}}.
  • Added RESTClient and Spring Boot tests for single and multiple annotations on allOf models, plus allOf-with-annotations.yaml.

• Refactors

  • Extracted normalizeVendorExtensionWithStringList and reused it for x-implements and x-class-extra-annotation normalization.

^{Written for commit 9db08fc. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 31 files

[jpfinne]

Merge the 2 tests

[jpfinne]

Use the supportedLibraries @DataProvider to test all libraries

[jpfinne]

merge the 2 tests

[jpfinne]

Create a reusable method, also available for other annotations and other generators (kotlin)

Maybe some static method: fixVendorExtensionWithStringList(Map<String, Object> vendorExtensions , String name) so you can use
fixVendorExtensionWithStringList(cm.getVendorExtensions(cm, "x-class-extra-annotation")

[jpfinne]

@johnyherangi Nice feature.

How can the user know that a list is now allowed for that specific annotation?
The description was corrct but with the wrong implementation: List of custom annotations to be added to model
Can you update it to reflect that both string and list are allowed?

Why only x-class-extra-annotation? There a at least 3 variants of x-xxx-extra-annotation