[BUG][java-helidon-server] generated code does not compile: interface expected here

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

i am trying to generate the server stubs for this yaml file:
https://github.com/simPod/GrafanaJsonDatasource/blob/0.6.x/openapi.yaml

using this maven code:

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>7.15.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/GrafanaJsonDatasource/openapi.yaml</inputSpec>
                            <generatorName>java-helidon-server</generatorName>
                            <library>mp</library>
                            <packageName>simPod.grafanaJsonDatasource.helidon</packageName>
                            <modelPackage>simPod.grafanaJsonDatasource.model</modelPackage>
                            <apiPackage>simPod.grafanaJsonDatasource.api</apiPackage>
                            <invokerPackage>simPod.grafanaJsonDatasource.invoker</invokerPackage>
                            <configOptions>
                                <!-- docs here: https://helidon.io/docs/v4/mp/openapi/openapi-generator -->
                                <fullProject>false</fullProject>
                                <disallowAdditionalPropertiesIfNotPresent>false</disallowAdditionalPropertiesIfNotPresent>
                                <enumUnknownDefaultCase>true</enumUnknownDefaultCase>
                                <legacyDiscriminatorBehavior>false</legacyDiscriminatorBehavior>
                                <useAbstractClass>true</useAbstractClass>
                                <useOneOfInterfaces>true</useOneOfInterfaces>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

but when i try and run: mvn compile

it gives me this error:

[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] /.../target/generated-sources/openapi/src/main/java/simPod/grafanaJsonDatasource/model/Dataframe.java:[29,36] interface expected here
[INFO] 1 error
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.264 s
[INFO] Finished at: 2025-09-03T18:52:48+01:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.14.0:compile (default-compile) on project XXX: Compilation failure
[ERROR] /.../target/generated-sources/openapi/src/main/java/simPod/grafanaJsonDatasource/model/Dataframe.java:[29,36] interface expected here

the 2 generated files causing the issue are:

package simPod.grafanaJsonDatasource.model;
...
public class Dataframe  implements ApiEndpointsVariable200Response {

Dataframe.java
but ApiEndpointsVariable200Response just a class:

package simPod.grafanaJsonDatasource.model;
...
public class ApiEndpointsVariable200Response  {

ApiEndpointsVariable200Response.java

@spericas @tjquinno @tvallin

openapi-generator version

7.15.0, (currently the latest version)

OpenAPI declaration file content or url

https://github.com/simPod/GrafanaJsonDatasource/blob/0.6.x/openapi.yaml

Generation Details

java/helidon/maven

Steps to reproduce

• create a simple Helidon MP project,
• add yaml file
• and maven snippet to from description
• try to compile project

Related issues/PRs

Suggest a fix

Reproduce the failure with the linked openapi.yaml, the java-helidon-server Maven configuration, and mvn compile. Start by comparing the generated Dataframe.java and ApiEndpointsVariable200Response.java declarations; done means the generated sources compile without the interface error, with coverage for this specification or configuration.