* [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
561e5bf9ab