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/tests/sdk.rs

@@ -116,6 +116,53 @@ fn discovery_and_dispatch() {
         .into();
     assert_eq!(internal.code, -32603);
 }
+#[sausage_plugin::test]
+async fn params_must_be_objects() {
+    for (params, kind) in [
+        (json!("{\"a\":2,\"b\":3}"), "a string"),
+        (json!([]), "an array"),
+        (json!(42), "a number"),
+        (json!(null), "null"),
+        (json!(true), "a boolean"),
+    ] {
+        let message = format!(
+            "params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string."
+        );
+        let outgoing = sausage_plugin::call("plugin.add", params.clone())
+            .await
+            .unwrap_err();
+        assert_eq!(outgoing.code, -32602);
+        assert_eq!(outgoing.message, message);
+        for method in ["add", "choice", "unknown"] {
+            let incoming = Plugin
+                .dispatch(method, params.clone(), &Ctx::default())
+                .await
+                .unwrap_err();
+            assert_eq!(incoming.code, -32602);
+            assert_eq!(incoming.message, message);
+        }
+    }
+    assert_eq!(
+        Plugin
+            .dispatch("add", json!({"a":2,"b":3}), &Ctx::default())
+            .await
+            .unwrap(),
+        5
+    );
+    assert_eq!(
+        Plugin
+            .dispatch("defaults", json!({}), &Ctx::default())
+            .await
+            .unwrap(),
+        json!({"value":""})
+    );
+    // An object reaches transport validation rather than the params rejection.
+    let outgoing = sausage_plugin::call("plugin.add", json!({"a":2,"b":3}))
+        .await
+        .unwrap_err();
+    assert!(outgoing.message.contains("no core transport"));
+}
+
 #[sausage_plugin::test]
 async fn missing_core_fails_without_wait() {
     assert!(