fix: Correct column list polars materialization engine (#5595)

fix: returning the correct column list when requesting columnsin the PolarsBackend

Signed-off-by: Dan Baron <dan.baron@starlingbank.com>

Author: danbaron63

sdk/python/feast/infra/compute_engines/local/backends/polars_backend.py

@@ -7,8 +7,8 @@
 
 
 class PolarsBackend(DataFrameBackend):
-    def columns(self, df):
-        pass
+    def columns(self, df: pl.DataFrame) -> list[str]:
+        return df.columns
 
     def from_arrow(self, table: pa.Table) -> pl.DataFrame:
         return pl.from_arrow(table)