fix: introspectToken() uses app token in AuthMode.APP

introspectToken() defaulted to the user access token (getAccessToken()), which
requires a token store and threw 'TokenStore is required' in app-only setups.
It now selects the token based on the configured AuthMode: app token for
AuthMode.APP, user token otherwise.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: TeksuSiK

src/main/java/pl/teksusik/kick4j/authorization/AuthorizationClient.java

@@ -229,9 +229,14 @@ public void revokeToken(String token, TokenTypeHint hint) {
 
     /**
      * Introspects the current managed access token via the OAuth introspection endpoint.
+     * Uses the app access token when the client is configured with {@link AuthMode#APP},
+     * otherwise the user access token.
      */
     public TokenIntrospect introspectToken() {
-        return this.introspectToken(this.getAccessToken());
+        String token = this.configuration.getAuthMode() == AuthMode.APP
+                ? this.getAppAccessToken()
+                : this.getAccessToken();
+        return this.introspectToken(token);
     }
 
     /**