Add backwards compatibly handling of sum stats of extension types (#7014)

After #6910, Older files with extension types will have zone stats that
are just based on the storage type (which might be arbitrary) and which
is rarely useful. we currently don't handle its existence at all, so
reading these files might panic or otherwise error. This PR falls back
to the storage type on the read path, but might not be the desirable
solution.

Signed-off-by: Adam Gutglick <adam@spiraldb.com>

Author: AdamGS

vortex-layout/src/layouts/zoned/zone_map.rs

@@ -80,6 +80,15 @@ impl ZoneMap {
                     .iter()
                     .filter_map(|stat| {
                         stat.dtype(column_dtype)
+                            .or_else(|| {
+                                // Backward compat: older files may have stored stats (e.g. Sum)
+                                // for extension types by resolving through the storage dtype.
+                                if let DType::Extension(ext) = column_dtype {
+                                    stat.dtype(ext.storage_dtype())
+                                } else {
+                                    None
+                                }
+                            })
                             .map(|dtype| (stat, dtype.as_nullable()))
                     })
                     .flat_map(|(s, dt)| match s {