Reduce invalidateCache() visibility from public to protected

Consistent with buildBundle() which is also @VisibleForTesting protected.

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

plugin/src/main/java/org/opensearch/sql/plugin/rest/RestPPLGrammarAction.java

@@ -86,7 +86,7 @@ protected GrammarBundle buildBundle() {
 
   /** Invalidate the cached bundle, forcing a rebuild on the next request. */
   @VisibleForTesting
-  public void invalidateCache() {
+  protected void invalidateCache() {
     synchronized (bundleLock) {
       cachedBundle = null;
     }