Name the received JSON type in non-object params errors

Keep -32602 and existing validation order; tell callers to send the object itself.
Cover host routing, stdout fast paths, SDK calls and generated dispatch.

Card: SAU-317

sdk/src/calls.rs

@@ -37,7 +37,7 @@ impl IntoFuture for Call {
     fn into_future(self) -> Self::IntoFuture {
         Box::pin(async move {
             if !self.params.is_object() {
-                return Err(Error::new(-32602, "params must be an object"));
+                return Err(Error::params_not_object(&self.params));
             }
             let (tx, rx) = oneshot::channel();
             let id = STATE.with(|s| {