fix: improve error handling and nil consistency in FormatArgsJSON

- Use fmt.Sprintf fallback instead of {} on encoding errors
- Normalize nil args to {} in FormatArgsJSON for consistent output
- Update tests to expect {} instead of null for nil args

Based on PR #2670 review feedback from afjcjsbx

pkg/agent/agent_utils.go

@@ -172,10 +172,6 @@ func toolFeedbackExplanationFromMessages(messages []providers.Message) string {
 }
 
 func toolFeedbackArgsPreview(args map[string]any, maxLen int) string {
-	if args == nil {
-		args = map[string]any{}
-	}
-
 	argsJSON := utils.FormatArgsJSON(args, true, false)
 	return utils.Truncate(argsJSON, maxLen)
 }