refactor(auth): 将 requestCache 字段修改为 final，增强类的不可变性

pigx-auth/src/main/java/com/pig4cloud/pigx/auth/support/handler/TenantSavedRequestAwareAuthenticationSuccessHandler.java

@@ -25,7 +25,7 @@ import java.io.IOException;
 @Slf4j
 public class TenantSavedRequestAwareAuthenticationSuccessHandler extends SimpleUrlAuthenticationSuccessHandler {
 
-	private RequestCache requestCache = new HttpSessionRequestCache();
+	private final RequestCache requestCache = new HttpSessionRequestCache();
 
 	public TenantSavedRequestAwareAuthenticationSuccessHandler() {
 	}