refactor: Update ClaudeResponse error handling to use pointer for ClaudeError and improve nil checks in response processing

dto/claude.go

@@ -183,7 +183,7 @@ type ClaudeResponse struct {
 	Completion   string               `json:"completion,omitempty"`
 	StopReason   string               `json:"stop_reason,omitempty"`
 	Model        string               `json:"model,omitempty"`
-	Error        ClaudeError          `json:"error,omitempty"`
+	Error        *ClaudeError         `json:"error,omitempty"`
 	Usage        *ClaudeUsage         `json:"usage,omitempty"`
 	Index        *int                 `json:"index,omitempty"`
 	ContentBlock *ClaudeMediaMessage  `json:"content_block,omitempty"`