agent: validate tool call arguments before dispatch

Reject tool calls with malformed JSON arguments early, returning
a clear error to the model instead of silently defaulting to null
and dispatching anyway. Prevents cascading failures when the model
generates truncated tool call arguments.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/agent/mod.rs

@@ -485,8 +485,16 @@ impl Agent {
         ui_tx: &UiSender,
         ds: &mut DispatchState,
     ) {
-        let args: serde_json::Value =
-            serde_json::from_str(&call.function.arguments).unwrap_or_default();
+        let args: serde_json::Value = match serde_json::from_str(&call.function.arguments) {
+            Ok(v) => v,
+            Err(e) => {
+                let err = format!("Error: malformed tool call arguments: {e}");
+                let _ = ui_tx.send(UiMessage::Activity(format!("rejected: {} (bad args)", call.function.name)));
+                let mut me = agent.lock().await;
+                me.apply_tool_result(call, err, ui_tx, ds);
+                return;
+            }
+        };
 
         let args_summary = summarize_args(&call.function.name, &args);
         let _ = ui_tx.send(UiMessage::Activity(format!("calling: {}", call.function.name)));