fix: specify exception type and condition in comment

resolve_names() raises KeyError (not a generic error) and only
validates when schema is present.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dtsong

data_diff/queries/ast_classes.py

@@ -515,7 +515,7 @@ def select(self, *exprs, **named_exprs) -> Self | ITable:
         named_exprs = _drop_skips_dict(named_exprs)
         exprs += _named_exprs_as_aliases(named_exprs)
         resolve_names(self.source_table, exprs)
-        # No explicit subset check needed; resolve_names() will raise if any column name is invalid
+        # No explicit subset check needed; resolve_names() raises KeyError for invalid column names (when schema is present)
         return attrs.evolve(self, columns=exprs)