fix(cr-2026-04-22): list_transcripts accepts omitted arguments

Regression surfaced by the batch review: commit 8400128 switched
list_transcripts from unwrap_or_default to map_err(-32602). This
correctly errors on malformed payloads but also rejected the common
case where a client omits the 'arguments' field entirely — which
arrives as Value::Null, and serde_json::from_value does not
deserialise Null into a struct.

Short-circuits the Null case to Args::default() before attempting
deserialisation. Genuine shape mismatches ("limit": "twenty")
still return -32602 as the previous test asserts.

New regression test: tools/call with list_transcripts and no
arguments key must return a successful response.

crates/mcp/src/lib.rs

@@ -191,13 +191,19 @@ async fn list_transcripts_tool(pool: &SqlitePool, args: Value) -> Result<Value,
         #[serde(default)]
         limit: Option<i64>,
     }
-    // Every other tool handler returns -32602 on malformed args;
-    // previously this one silently fell back to defaults, hiding
-    // client bugs (2026-04-22 review MAJOR). An empty Value
-    // deserialises cleanly into Args::default, so the "no args"
-    // path still works — only genuinely malformed payloads error.
-    let args: Args = serde_json::from_value(args)
-        .map_err(|e| error(-32602, format!("Invalid arguments: {e}")))?;
+    // The `arguments` field in CallParams defaults to `Value::Null`
+    // when a client omits it entirely. `serde_json::from_value` does
+    // not accept Null as an empty object, so we short-circuit that
+    // case before deserialising — a missing `arguments` still falls
+    // back to defaults (the common case for list_transcripts), while
+    // a genuinely malformed payload returns -32602 per the Invalid
+    // arguments contract the other handlers use.
+    let args: Args = if args.is_null() {
+        Args::default()
+    } else {
+        serde_json::from_value(args)
+            .map_err(|e| error(-32602, format!("Invalid arguments: {e}")))?
+    };
     let limit = args.limit.unwrap_or(20).clamp(1, 200);
 
     let rows = kon_storage::list_transcripts(pool, limit)
@@ -426,6 +432,34 @@ mod tests {
         assert!(err.message.contains("expected value"));
     }
 
+    #[tokio::test]
+    async fn list_transcripts_accepts_omitted_arguments() {
+        // Regression for the review-of-review: tools/call requests
+        // that omit `arguments` arrive with `Value::Null`. The
+        // malformed-params fix must not reject those — it is the
+        // common shape for an empty call, equivalent to defaults.
+        let request = json!({
+            "jsonrpc": "2.0",
+            "id": 98,
+            "method": "tools/call",
+            "params": {
+                "name": "list_transcripts",
+                // `arguments` omitted
+            },
+        });
+
+        let pool = sqlx::SqlitePool::connect("sqlite::memory:").await.unwrap();
+        kon_storage::migrations::run_migrations(&pool).await.unwrap();
+        let response = handle_message(&pool, request).await.expect("has response");
+
+        assert!(
+            response.error.is_none(),
+            "omitted arguments must not error, got: {:?}",
+            response.error
+        );
+        assert!(response.result.is_some());
+    }
+
     #[tokio::test]
     async fn list_transcripts_rejects_malformed_params_with_invalid_arguments() {
         // Regression for the 2026-04-22 review MAJOR: previously the