rpc (fix): Fix generated client interface (#3830)

Author: xerial

airframe-http-codegen/src/main/scala/wvlet/airframe/http/codegen/HttpClientIR.scala

@@ -175,6 +175,13 @@ object HttpClientIR extends LogSupport {
   ) extends ClientCodeIR {
     def typeArgString =
       typeArgs
+        .map(arg =>
+          if (arg.rawType.isAssignableFrom(classOf[Rx[_]]) && arg.typeArgs.size > 0)
+            // Extract Rx element type
+            arg.typeArgs(0)
+          else
+            arg
+        )
         .map(arg => HttpClientGenerator.fullTypeNameOf(arg))
         .mkString(", ")
     def clientMethodName = {

airframe-http-codegen/src/main/scala/wvlet/airframe/http/codegen/client/RPCClientGenerator.scala

@@ -164,7 +164,7 @@ object RPCClientGenerator extends HttpClientGenerator {
             m.httpMethod match {
               case HttpMethod.GET =>
                 s"""def ${m.name}(${inputArgs.mkString(", ")}): ${returnType} = {
-                   |  client.readAs[${m.returnType.fullTypeName}](Http.GET(s"${m.path}"))
+                   |  client.readAs[${returnElementType}](Http.GET(s"${m.path}"))
                    |}""".stripMargin
               case _ =>
                 val args = Seq.newBuilder[String]