fix: add field count validation in AVRO_RECORD encoding

Validate that the number of fields matches between Arrow struct array,
Iceberg struct type, and Avro node before iterating to prevent
out-of-bounds access.

- Check struct_array.num_fields() matches avro_node->leaves()
- Check struct_type.fields().size() matches avro_node->leaves()
- Return InvalidArgument error with clear message on mismatch

Author: shangxinli

src/iceberg/avro/avro_direct_encoder.cc

@@ -245,6 +245,18 @@ Status EncodeArrowToAvro(const ::avro::NodePtr& avro_node, ::avro::Encoder& enco
       const auto& struct_type = static_cast<const StructType&>(type);
       const size_t num_fields = avro_node->leaves();
 
+      // Validate field count matches
+      if (struct_array.num_fields() != static_cast<int>(num_fields)) {
+        return InvalidArgument(
+            "Field count mismatch: Arrow struct has {} fields, Avro node has {} fields",
+            struct_array.num_fields(), num_fields);
+      }
+      if (struct_type.fields().size() != num_fields) {
+        return InvalidArgument(
+            "Field count mismatch: Iceberg struct has {} fields, Avro node has {} fields",
+            struct_type.fields().size(), num_fields);
+      }
+
       for (size_t i = 0; i < num_fields; ++i) {
         const auto& field_node = avro_node->leafAt(i);
         const auto& field_array = struct_array.field(static_cast<int>(i));