forked from loafle/openapi-generator-original
* [kotlin][jvm-okhttp4] Fix multipart/form-data with JSON content-type Fixes #16457 Fixes two critical bugs in multipart/form-data handling when parts have Content-Type application/json: 1. IllegalArgumentException: OkHttp throws "Unexpected header: Content-Type" because Content-Type was passed in headers map instead of via asRequestBody(mediaType)/toRequestBody(mediaType) parameter. 2. Invalid JSON serialization: Non-file parts with application/json Content-Type were serialized using toString() instead of proper JSON serialization, producing invalid output like: "MyObject(field1=value, field2=123)" instead of '{"field1":"value","field2":123}' Changes: - Filter Content-Type from headers before passing to OkHttp - Check part Content-Type and use appropriate serializer (JSON vs toString) - Add integration tests with echo server to verify fix - Support all serialization libraries (gson, moshi, jackson, kotlinx) Fixes issues with multipart endpoints that mix file uploads with JSON metadata, common in REST APIs for document/image uploads. * Run mvn clean/package, and regenerate samples * Add fix for kotlinx serialisation issue * Refactor multipart helpers for reified type parameter support * Fix kotlinx.serialization multipart by adding serializer lambda to PartConfig * Fix internal Ktor API usage in multipart forms