fix: filter the vec index in function Index::scalar_index_info (#3000)

Create the vector index error when the schema has nullable field.


![image](https://github.com/user-attachments/assets/d323b9e0-6d39-4293-92b7-3ba3adfc8b2b)

When the vector column had build index already, the dataset scaner will
assign the filter in function `compute_partitions`.
```python
    if dataset.schema.field(column).nullable and filter_nan:
        filt = f"{column} is not null"
    else:
        filt = None
```

The scanner will use the scalar index scanner and this scanner treat the
vector index as scalar index(btree index).

This pr will filter the vector index in `scalar_index_info` and it will
only return the scalar index now.

rust/lance/src/index.rs

@@ -781,7 +781,14 @@ impl DatasetIndexInternalExt for Dataset {
         let indices = self.load_indices().await?;
         let schema = self.schema();
         let mut indexed_fields = Vec::new();
-        for index in indices.iter().filter(|idx| idx.fields.len() == 1) {
+        for index in indices.iter().filter(|idx| {
+            let idx_schema = schema.project_by_ids(idx.fields.as_slice());
+            let is_vector_index = idx_schema
+                .fields
+                .iter()
+                .any(|f| matches!(f.data_type(), DataType::FixedSizeList(_, _)));
+            idx.fields.len() == 1 && !is_vector_index
+        }) {
             let field = index.fields[0];
             let field = schema.field_by_id(field).ok_or_else(|| Error::Internal {
                 message: format!(