ArcadeData#2915 fix: ensure Jvector HNSW graph file is closed and flushed to disk on database close (ArcadeData#2916)

* ArcadeData#2915 fix: ensure Jvector  HNSW graph file is closed and flushed to disk on database close

* fix: address PR ArcadeData#2916 review comments for HNSW graph persistence

Improvements to code quality and maintainability:

* Enhanced exception logging: Include full stack trace when closing graph file
  for better debugging. Changed from logging only the error message to passing
  the exception object to LogManager for complete context.

* Refactored deleteDirectory() helper: Replaced manual recursive directory
  traversal with modern java.nio.file.Files.walk() API. This approach is more
  robust, efficient, and follows Java best practices for file tree operations.
  - Uses try-with-resources for proper resource management
  - Sorts in reverse order to delete files before directories
  - Provides better exception handling with IOException

All existing tests continue to pass (22/22).

Addresses review comments from PR ArcadeData#2916:
- ArcadeData#2916 (comment)
- ArcadeData#2916 (comment)

* ArcadeData#2915 fix: ensure Jvector  HNSW graph file is closed and flushed to disk on database close

Author: robfrank

engine/src/main/java/com/arcadedb/index/vector/LSMVectorIndex.java

@@ -491,9 +491,12 @@ private LSMVectorIndexGraphFile discoverAndLoadGraphFile() {
       final DatabaseInternal database = getDatabase();
       final String expectedGraphFileName = mutable.getName() + "_" + LSMVectorIndexGraphFile.FILE_EXT;
 
+      LogManager.instance().log(this, Level.FINE,
+          "Discovering graph file for index %s, looking for: %s", indexName, expectedGraphFileName);
+
       // Look for ComponentFile in FileManager
       for (final ComponentFile file : database.getFileManager().getFiles()) {
-        if (LSMVectorIndexGraphFile.FILE_EXT.equals(file.getFileExtension()) &&
+        if (file != null && LSMVectorIndexGraphFile.FILE_EXT.equals(file.getFileExtension()) &&
             file.getComponentName().equals(expectedGraphFileName)) {
 
           final int pageSize = file instanceof com.arcadedb.engine.PaginatedComponentFile ?
@@ -513,6 +516,8 @@ private LSMVectorIndexGraphFile discoverAndLoadGraphFile() {
         }
       }
 
+      LogManager.instance().log(this, Level.FINE,
+          "No graph file found in FileManager for index %s. Graph will be built on first search.", indexName);
       return null;
     } catch (final Exception e) {
       LogManager.instance().log(this, Level.WARNING,
@@ -2129,6 +2134,16 @@ public void close() {
         // to avoid path transformation issues during replication.
 
         mutable.close();
+
+        // Close graph file to ensure graph data is flushed to disk
+        if (graphFile != null) {
+          try {
+            graphFile.close();
+          } catch (final Exception e) {
+            LogManager.instance().log(this, Level.WARNING,
+                "Error closing graph file for index %s", e, indexName);
+          }
+        }
       } finally {
         lock.writeLock().unlock();
       }