[BUG] [JAVA] [NATIVE] api.moustache asyncNative is missing vendorExtensions.x-java-text-plain-string code

• Have you provided a full/minimal spec to reproduce the issue?

  "paths" : {
    "/" : {
      "get" : {
        "tags" : [ "download" ],
        "operationId" : "getFile",
        "responses" : {
          "200" : {
            "description" : "OK",
            "content" : {
              "text/plain" : {
                "schema" : {
                  "type" : "string"
                }
              }
            }
          }
        }
      }
    }

• Have you tested with the latest master to confirm the issue still exists?
  Fails against 7.0.0-beta
• What's the actual output vs expected output?
  Code tries to parse string as a json object

Description

Trying to return a file but the result kept being parsed as a json response.
Discovered I needed to use <asyncNative>false</asyncNative> in order for vendorExtensions.x-java-text-plain-string
to work

openapi-generator version

7.0.0-beta

Steps to reproduce

Run with "nativeAsync" option

Related issues/PRs

Result of #14132

Suggest a fix

Replicate non-async code section in async secion

Start in the api.moustache template and compare the nativeAsync section with the non-async section that handles vendorExtensions.x-java-text-plain-string. Reproduce the provided text/plain response with the nativeAsync option and verify that generated code no longer parses the string as a JSON object.