fix(core): add synchronization to addLink to prevent race conditions

Add synchronized keyword to addLink method to ensure thread safety.
The method now performs compound operations (check -> add -> check -> rollback)
which could lead to race conditions without proper synchronization.

This aligns with the existing synchronized hasLink method.

Co-authored-by: hsluoyz <3787410+hsluoyz@users.noreply.github.com>

Author: Copilot

src/main/java/org/casbin/jcasbin/rbac/DefaultRoleManager.java

@@ -168,7 +168,7 @@ public void clear() {
      * inherits role: name2. domain is a prefix to the roles.
      */
     @Override
-    public void addLink(String name1, String name2, String... domain) {
+    public synchronized void addLink(String name1, String name2, String... domain) {
         Role user = getRole(name1);
         Role role = getRole(name2);