fix: keep oneOf when WRAPPER_OBJECT wrapping is applied

[vpelikh]

Summary

A schema that already carries a oneOf now keeps it when Jackson's WRAPPER_OBJECT type info is applied. Previously the composition was silently dropped and the schema became a plain object with a single property.

Problem

ModelResolver.resolveWrapping(...) builds a new wrapper schema and nests the model inside it:

Schema wrapperSchema = openapi31 ? new JsonSchema().typesItem("object") : new ObjectSchema();
wrapperSchema.name(model.getName());
wrapperSchema.addProperties(name, model);
return wrapperSchema;

The wrapper copies only the name and the single property. If model had a oneOf (from @Schema(oneOf = ...), @Schema(subTypes = ...) or @JsonSubTypes), it is lost: the returned schema is an ObjectSchema, not a ComposedSchema, and the oneOf is gone.

Reproduction (@JsonTypeInfo(include = WRAPPER_OBJECT) + @Schema(oneOf)):

	result
before	ObjectSchema, properties [Pet], no oneOf
after	ComposedSchema, oneOf = 2 refs, wrapper property present

Change

When the model already has a non-empty oneOf, add the wrapper property directly to the model and return it, instead of nesting it in a wrapper schema:

if (model.getOneOf() != null && !model.getOneOf().isEmpty()) {
    model.addProperties(name, newObjectSchema());
    return model;
}

The empty-object-schema expression was already used once in this method, so instead of repeating it a second time it is extracted to a small helper:

/** An empty object schema for the current OpenAPI version. */
private Schema newObjectSchema() {
    return openapi31 ? new JsonSchema().typesItem("object") : new ObjectSchema();
}

Behavior is unchanged for models without oneOf.

Tests

Adds WrapperObjectOneOfTest. It fails before the change (ObjectSchema with no oneOf) and passes after it.

Notes

This is the small stack split out of #5320. It is a standalone bug fix, reproducible on unmodified 3.0.0, and does not depend on @JsonSubTypes work.