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| {

sdk/src/error.rs

@@ -15,6 +15,25 @@ pub struct Error {
     pub data: Option<Value>,
 }
 impl Error {
+    /// Build the object-params diagnostic used by calls and generated dispatch.
+    #[doc(hidden)]
+    pub fn params_not_object(params: &Value) -> Self {
+        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",
+        };
+        Self::new(
+            -32602,
+            format!(
+                "params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string."
+            ),
+        )
+    }
+
     /// Construct an error without structured details.
     pub fn new(code: i64, message: impl Into<String>) -> Self {
         Self {