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

cli/src/routing.rs

@@ -7,6 +7,23 @@ use serde_json::{Value, json};
 use tokio::sync::{mpsc, oneshot, watch};
 use uuid::Uuid;
 
+fn params_not_object(params: &Value) -> Value {
+    let kind = match params {
+        Value::String(_) => "a string",
+        Value::Array(_) => "an array",
+        Value::Number(_) => "a number",
+        Value::Null => "null",
+        Value::Bool(_) => "a boolean",
+        Value::Object(_) => "an object",
+    };
+    error(
+        -32602,
+        &format!(
+            "params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string."
+        ),
+    )
+}
+
 impl Host {
     /// Queue an incoming response or notification for its destination.
     pub(crate) fn send(&self, pid: &str, value: Value) {
@@ -69,7 +86,7 @@ impl Host {
         let host = self.clone();
         Box::pin(async move {
             if !params.is_object() {
-                return Err(error(-32602, "params must be an object"));
+                return Err(params_not_object(&params));
             }
             if caller != "console" && caller != "core" {
                 let state = host.0.state.lock().unwrap();
@@ -238,7 +255,7 @@ impl Host {
                 // Deliver progress before a following response removes its destination.
                 let params = v.get("params").cloned().unwrap_or(json!({}));
                 let result = if !params.is_object() {
-                    Err(error(-32602, "params must be an object"))
+                    Err(params_not_object(&params))
                 } else if method == "core.unlock" {
                     if let Some(path) = params["path"].as_str() {
                         self.unlock(pid, path);