append_new_columns doesn't allow you to add new columns to a model

### Describe the bug
`on_schema_change` is set to `append_new_columns` but when I add new columns, I get a NUMBER_OF_COLUMNS_DOESNT_MATCH error:

```
Database Error in model xxxxxxxxx (xxxxxxxxx)
--
  | HTTPDriver for xxxxxxxxx received ClickHouse error code 20
  | Code: 20. DB::Exception: Number of columns doesn't match (source: 15 and result: 14). (NUMBER_OF_COLUMNS_DOESNT_MATCH) (version 24.10.1.11366 (official build))
  | compiled code at xxxxxxxxxxx
```

I fixed this by going into Clickhouse directly and writing an `ALTER TABLE` query to add the new column. Then when I re materialised the asset, it worked

### Steps to reproduce
1. create an incremental model with `on_schema_change` = `append_new_columns`
2. materialise it
3. add a new column
4. materialise it again and you should see the above error

### Expected behaviour
When `on_schema_change` is set to `append_new_columns` and you add a new column to an already materialised model then it should allow you to add those columns

Start by reproducing the incremental model workflow described: materialise the model, add a column, and materialise it again with on_schema_change set to append_new_columns. Then trace the implementation of that schema-change path and verify that the existing table gains the new column without a NUMBER_OF_COLUMNS_DOESNT_MATCH error.

Written by the indexing model from the issue text.