[BUG][JAVA][SPRING] referencing multiple external files in one OpenAPI file results in broken builds or code

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

Currently, I'm working on migrating our old swagger 2.0 APIs to OpenAPI 3.0.1.
The migration process itself runs pretty smoothly.

Unfortunately, I've encountered many issues with OpenAPI 3.0.1 files referencing external YAML files (using relative paths) and it's relatively difficult to describe all of them within this ticket.
Therefore I created a GitHub repository for all the problems I've encountered (so far)

https://github.com/mlemnian/openapi-generator-issues

openapi-generator version

Tested with 6.4.0, 6.5.0 and 6.6.0-SNAPSHOT at 3d7c173eb2f40ff97d067eaa14d2991f2cd6b7a5

OpenAPI declaration file content or URL

I've created a dedicated GitHub repository for all the problems I've encountered (so far)

https://github.com/mlemnian/openapi-generator-issues

Generation Details

In my examples, I'm using always the following configs:

• maven 3.9.0
• java 17

                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <generatorName>spring</generatorName>
                            <generateSupportingFiles>false</generateSupportingFiles>
                            <generateAliasAsModel>false</generateAliasAsModel>
                            <inputSpec>${project.basedir}/open-api.yaml</inputSpec>
                            <modelPackage>api.model</modelPackage>
                            <modelNameSuffix>DTO</modelNameSuffix>
                            <apiPackage>api</apiPackage>
                            <skipValidateSpec>false</skipValidateSpec>
                            <configOptions>
                                <useSpringBoot3>true</useSpringBoot3>
                                <performBeanValidation>true</performBeanValidation>
                                <interfaceOnly>true</interfaceOnly>
                                <skipDefaultInterface>true</skipDefaultInterface>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>

Steps to reproduce

follow the description in the repo examples

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/11151
https://github.com/OpenAPITools/openapi-generator/issues/3848
https://github.com/OpenAPITools/openapi-generator/issues/1976
https://github.com/OpenAPITools/openapi-generator/issues/8844

Suggest a fix

Start with the examples in the linked openapi-generator-issues repository and the referenced open-api.yaml, then run the Maven generation configuration using the spring generator. Compare the generated output and build results when multiple external YAML files are referenced; done means the documented examples generate and build successfully.