[BUG][JAVA][SPRING] getNativeResponse could return null and must be enforced

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

With Java & spring generator, when using JSpecify Nullable annotation.
Nullaway create a compilation error because the generated code is not nullsafe for ApiUtil.

openapi-generator version

7.18.2, this is not a regression

OpenAPI declaration file content or url

My Maven plugin conf:

                    <configuration>
                        <generatorName>spring</generatorName>
...
                        <importMappings>
                            <importMapping>Nullable=org.jspecify.annotations.Nullable</importMapping>
                        </importMappings>
                    </configuration>

Generation Details

The generated code is:

public class ApiUtil {
    public static void setExampleResponse(NativeWebRequest req, String contentType, String example) {
        try {
            HttpServletResponse res = req.getNativeResponse(HttpServletResponse.class); /* <== could be null because getNativeResponse return @Nullable */
            res.setCharacterEncoding("UTF-8");
            res.addHeader("Content-Type", contentType);
            res.getWriter().print(example);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

The code should be:

public class ApiUtil {
    public static void setExampleResponse(NativeWebRequest req, String contentType, String example) {
        try {
            HttpServletResponse res = req.getNativeResponse(HttpServletResponse.class); /* <== could be null because getNativeResponse return @Nullable */
            if (res != null) {
                res.setCharacterEncoding("UTF-8");
                res.addHeader("Content-Type", contentType);
                res.getWriter().print(example);
              }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

Steps to reproduce

Setup JSpecify (like in my pom example) and NullAway.
Run the generator, when compiling the following Warns are generated:

[WARNING] .../target/generated-sources/src/main/java/tech/generated/api/ApiUtil.java:[12,16] [NullAway] dereferenced expression res is @Nullable
    (see http://t.uber.com/nullaway )

Related issues/PRs

New bugs detected with the generalisation of JSpecify in Spring and other frameworks.
https://github.com/OpenAPITools/openapi-generator/issues/22598

Suggest a fix

See PR.

Inspect the Spring generator template or entry point responsible for emitting ApiUtil, using the shown Maven configuration and generated method as the starting point. Reproduce generation with JSpecify and NullAway, then verify that the generated response handling compiles without a nullable dereference warning.