KSRPC_ERROR_CODE_HEADER

Header carrying the original ksrpc error code when the wire status maps to the default 500 fallback (i.e. the code is not present in the configured errorCodeToHttpStatus map). The client uses this to recover the exact code for @KsError-bound payload routing on the receive path. Match this constant on both ends — see also the duplicate definition in ksrpc-ktor-server's HttpStream.kt.