fix: add child field conversion support in JsonArrowSchemaConverter (#187)

Fixes a critical bug in `JsonArrowSchemaConverter` where child fields
for complex Arrow types (list, struct, map) were not being converted,
causing "Lists have one child Field. Found: none" errors when creating
Lance tables with array columns.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: LuQQiu

java/lance-namespace-core/src/main/java/com/lancedb/lance/namespace/util/JsonArrowSchemaConverter.java

@@ -65,7 +65,18 @@ private static Field convertToArrowField(JsonArrowField jsonField) {
         jsonField.getMetadata() != null ? jsonField.getMetadata() : new HashMap<>();
 
     FieldType fieldType = new FieldType(nullable, arrowType, null, metadata);
-    return new Field(name, fieldType, null);
+
+    // Convert child fields if they exist (needed for list, struct, map, etc.)
+    List<Field> children = null;
+    boolean hasChildFields = jsonField.getType() != null && jsonField.getType().getFields() != null;
+    if (hasChildFields) {
+      children = new ArrayList<>();
+      for (JsonArrowField childField : jsonField.getType().getFields()) {
+        children.add(convertToArrowField(childField));
+      }
+    }
+
+    return new Field(name, fieldType, children);
   }
 
   private static ArrowType convertToArrowType(JsonArrowDataType jsonType) {