[BUG][SPRING] Using useBeanValidation together with openApiNullable creates model classes that validate null values incorrectly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I have a spec that contains a repsonse that contains a json object with a nullable required field (see below). When I use the spring generator with useBeanValidation: true and openApiNullable: true (they are enabled per default) it produces a model that contains the following:

public class Config {
  @JsonProperty("endpoint")
  private JsonNullable<String> endpoint = JsonNullable.undefined();

  public Config endpoint(String endpoint) {
    this.endpoint = JsonNullable.of(endpoint);
    return this;
  }

  @ApiModelProperty(required = true, value = "")
  @NotNull
  public JsonNullable<String> getEndpoint() {
    return endpoint;
  }

  public void setEndpoint(JsonNullable<String> endpoint) {
    this.endpoint = endpoint;
}

When I create an instance of this model where the field endpoint is null and try to validate it, the validator says it is invalid because endpoint must not be null:

var config = new Config().setEndpoint(null);
var validator = Validation.buildDefaultValidatorFactory().getValidator();
var validationResult = validator.validate(config);

I think this happens because JsonNullable is implemented in a way that the validation constraint is applied to the wrapped value and not to the JsonNullable object itself (see https://github.com/OpenAPITools/jackson-databind-nullable)

openapi-generator version

5.3.1
I cannot upgrade to 5.4.0 because of https://github.com/OpenAPITools/openapi-generator/issues/11506

OpenAPI declaration file content or url

components:
  responses:
    Config:
      description: success
      content:
        application/json:
          schema:
            title: Config
            type: object
            properties:
              endpoint:
                type: string
                nullable: true
            required:
              - endpoint

Generation Details

Using the spring generator with useBeanValidation: true and openApiNullable: true

Suggest a fix

Do not apply @NotNull annotation to getters that return a JsonNullable object.

Start with the Spring generator's handling of useBeanValidation and openApiNullable, then reproduce the issue with the Config model shown in the report. Confirm that validation incorrectly rejects a null endpoint and that the completed change avoids applying @NotNull to getters returning JsonNullable while preserving required-field validation elsewhere.