fix(dispatcher): WW-3576 remove redundant volatile from SessionMap fields

The volatile keyword on non-primitive fields (HttpSession, Set) triggers
Sonar rule S3077 because volatile only guarantees reference visibility,
not thread-safety of object contents.

Since all field accesses are already protected by synchronized(this),
the volatile keyword is redundant - synchronization already provides
both visibility (happens-before) and atomicity guarantees.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lukaszlenart

core/src/main/java/org/apache/struts2/dispatcher/SessionMap.java

@@ -39,8 +39,8 @@ public class SessionMap extends AbstractMap<String, Object> implements Serializa
     @Serial
     private static final long serialVersionUID = 4678843241638046854L;
 
-    protected volatile HttpSession session;
-    protected volatile Set<Entry<String, Object>> entries;
+    protected HttpSession session;
+    protected Set<Entry<String, Object>> entries;
     protected HttpServletRequest request;