Address review comments

Author: KD23243

components/org.wso2.carbon.identity.oauth/src/main/java/org/wso2/carbon/identity/oauth2/OAuth2Constants.java

@@ -34,6 +34,17 @@ public static class TokenBinderType {
         public static final String CLIENT_REQUEST = "client-request";
 
     }
+
+    /**
+     * Constants for token types.
+     */
+    public static class TokenTypes {
+
+        public static final String OPAQUE = "Opaque";
+        public static final String JWT = "jwt";
+
+    }
+
     public static final String GROUPS = "groups";
     public static final String ENTITY_ID = "entity_id";
     public static final String IS_CONSENTED = "is_consented";

components/org.wso2.carbon.identity.oauth/src/main/java/org/wso2/carbon/identity/oauth2/token/OauthTokenIssuer.java

@@ -24,6 +24,8 @@
 import org.wso2.carbon.identity.oauth2.IdentityOAuth2Exception;
 import org.wso2.carbon.identity.oauth2.authz.OAuthAuthzReqMessageContext;
 
+import static org.wso2.carbon.identity.oauth2.OAuth2Constants.TokenTypes.OPAQUE;
+
 /**
  * OAuth 2 access token issuer.
  */
@@ -106,6 +108,7 @@ default String issueSubjectToken(OAuthAuthzReqMessageContext oauthAuthzMsgCtx) t
      * @return The type of the token (e.g., "JWT" or "Opaque").
      */
     default String getAccessTokenType() {
-        return "Opaque";
+
+        return OPAQUE;
     }
 }