Set word2vec getSynonyms method synchronized.

This is a quick mitigation to avoid unexpected behaviour if multiple queries
are executed concurrently.

This commit wants to address some comments received in the PR apache#12169

lucene/analysis/common/src/java/org/apache/lucene/analysis/synonym/word2vec/Word2VecSynonymProvider.java

@@ -42,6 +42,7 @@ public class Word2VecSynonymProvider {
   private static final VectorSimilarityFunction SIMILARITY_FUNCTION =
       VectorSimilarityFunction.DOT_PRODUCT;
   private static final VectorEncoding VECTOR_ENCODING = VectorEncoding.FLOAT32;
+  private static final int NO_LIMIT_ON_VISITED_NODES = Integer.MAX_VALUE;
   private final Word2VecModel word2VecModel;
   private final HnswGraph hnswGraph;
 
@@ -64,7 +65,16 @@ public Word2VecSynonymProvider(Word2VecModel model) throws IOException {
     this.hnswGraph = builder.build(word2VecModel.copy());
   }
 
-  public List<TermAndBoost> getSynonyms(
+  /**
+   * Returns the list of synonyms of a provided term.
+   * This method is synchronized because it uses the {@link org.apache.lucene.util.hnsw.OnHeapHnswGraph}
+   * that is not thread-safe.
+   *
+   * @param term term to search to find synonyms
+   * @param maxSynonymsPerTerm limit of synonyms returned
+   * @param minAcceptedSimilarity lower similarity threshold to consider another term as synonym
+   */
+  public synchronized List<TermAndBoost> getSynonyms(
       BytesRef term, int maxSynonymsPerTerm, float minAcceptedSimilarity) throws IOException {
 
     if (term == null) {
@@ -85,7 +95,7 @@ public List<TermAndBoost> getSynonyms(
               SIMILARITY_FUNCTION,
               hnswGraph,
               null,
-              word2VecModel.size());
+              NO_LIMIT_ON_VISITED_NODES);
 
       int size = synonyms.size();
       for (int i = 0; i < size; i++) {