fix(agent): log warning when native tool call arguments fail JSON parsing

The NativeToolDispatcher silently defaults to an empty object when tool
call arguments from the LLM fail to parse as JSON. The XML dispatcher
already logs a warning for the same case (line 68). Add a matching
tracing::warn with tool name and parse error for observability parity.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/agent/dispatcher.rs

@@ -166,8 +166,14 @@ impl ToolDispatcher for NativeToolDispatcher {
             .iter()
             .map(|tc| ParsedToolCall {
                 name: tc.name.clone(),
-                arguments: serde_json::from_str(&tc.arguments)
-                    .unwrap_or_else(|_| Value::Object(serde_json::Map::new())),
+                arguments: serde_json::from_str(&tc.arguments).unwrap_or_else(|e| {
+                    tracing::warn!(
+                        tool = %tc.name,
+                        error = %e,
+                        "Failed to parse native tool call arguments as JSON; defaulting to empty object"
+                    );
+                    Value::Object(serde_json::Map::new())
+                }),
                 tool_call_id: Some(tc.id.clone()),
             })
             .collect();