[BUG][JAVA] oneOf creates non-compilable model

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

Spec: https://docs.btcpayserver.org/API/Greenfield/v1/swagger.json

org.openapitools.client.model.InvoiceMetadata model contains an invalid import:

import org.openapitools.client.model.OneOfstringobject;

The above class doesn't exist.

openapi-generator version

6.4.0 and 6.5.0

OpenAPI declaration file content or url

https://docs.btcpayserver.org/API/Greenfield/v1/swagger.json

Generation Details

java -jar ~/Downloads/openapi-generator-cli-6.5.0-20230219.124821-1.jar generate -i https://docs.btcpayserver.org/API/Greenfield/v1/swagger.json -g java -o . --skip-validate-spec

Steps to reproduce

java -jar ~/Downloads/openapi-generator-cli-6.5.0-20230219.124821-1.jar generate -i https://docs.btcpayserver.org/API/Greenfield/v1/swagger.json -g java -o . --skip-validate-spec

Open the generated project on IDE and see the compilation error in org.openapitools.client.model.InvoiceMetadata class.

Related issues/PRs

Suggest a fix

Start by running the provided openapi-generator command with the linked BTCPay Server swagger.json and inspect the generated InvoiceMetadata model. Trace how the Java generator handles the oneOf schema and creates imports. Done means the generated project compiles and no longer imports the nonexistent OneOfstringobject class.