fix: propagate IsError in parallel tool execution path

The squash merge of #291 missed the parallel tool execution path.
Tool errors running in parallel still displayed as "Done" instead
of "Failed" when reloading chat history.

internal/agent/loop.go

@@ -1193,6 +1193,7 @@ func (l *Loop) runLoop(ctx context.Context, req RunRequest) (*RunResult, error)
 					Role:       "tool",
 					Content:    r.result.ForLLM,
 					ToolCallID: r.tc.ID,
+					IsError:    r.result.IsError,
 				}
 				messages = append(messages, toolMsg)
 				pendingMsgs = append(pendingMsgs, toolMsg)