Disallow fragment scan on index query.

rust/lance/src/dataset/scanner.rs

@@ -867,6 +867,10 @@ impl Scanner {
             // This tests if any of the fragments are missing the physical_rows property (old style)
             // If they are then we cannot use scalar indices
             if filter_plan.index_query.is_some() {
+                // Index scan doesn't honor the fragment allowlist today.
+                // TODO: we could filter the index scan results to only include the allowed fragments.
+                self.ensure_not_fragment_scan()?;
+
                 let fragments = if let Some(fragments) = self.fragments.as_ref() {
                     fragments
                 } else {