fix(auth): make clientSecret optional for community mode

clientId is now the only required credential for features requiring auth.
clientSecret remains optional for community/self-hosted deployments
but is still validated server-side for enterprise mode.

Part of OAuth2 migration - PR #948

Author: saurabhjain1592

src/main/java/com/getaxonflow/sdk/AxonFlow.java

@@ -1675,12 +1675,12 @@ private void addAuthHeaders(Request.Builder builder) {
      * Requires credentials for enterprise features.
      *
      * @param feature the feature name for error message
-     * @throws AuthenticationException if no credentials are configured
+     * @throws AuthenticationException if clientId is not configured
      */
     private void requireCredentials(String feature) {
         if (!config.hasCredentials()) {
             throw new AuthenticationException(
-                feature + " requires credentials. Set clientId and clientSecret in config."
+                feature + " requires clientId. Set clientId in config (clientSecret is optional for community mode)."
             );
         }
     }

src/main/java/com/getaxonflow/sdk/AxonFlowConfig.java

@@ -85,13 +85,13 @@ private void validate() {
     /**
      * Checks if credentials are configured.
      *
-     * <p>Returns true if client credentials (clientId and clientSecret) are set.
+     * <p>Returns true if clientId is set.
+     * clientSecret is optional for community mode but required for enterprise.
      *
-     * @return true if credentials are available
+     * @return true if clientId is available
      */
     public boolean hasCredentials() {
-        return clientId != null && !clientId.isEmpty() &&
-               clientSecret != null && !clientSecret.isEmpty();
+        return clientId != null && !clientId.isEmpty();
     }
 
     private String normalizeUrl(String url) {