[BUG] [java][native] Encoding failure due to last change of empty response handling

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?

Description

The change in api.mustache to use a new String instead of passing the InputStream direct to the ObjectMapper causes an encoding problem. ObjectMapper always uses UTF-8 for encoding. The Constructor String(byte[] bytes) uses the default Characterset of the running virtual machine.

openapi-generator version

7.11.0

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/8c337f05bef92b207353269649f63f5002ae1cb0/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache#L292

Steps to reproduce

use a native client in a java app that is running with a default encoding, that is not UTF-8

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/20334

Suggest a fix

Revert the usage of String(byte[] bytes) or use public String(byte[] bytes, Charset charset)

Start with modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache at the referenced line, and review the response handling introduced by the linked change in PR 20334. Update the byte-to-string handling so it does not depend on the JVM's default charset, then verify that a native client works when the default encoding is not UTF-8.