Update column names in pdsh query 18 (#21119)

This updates the column names in query 18 to match other engines.

Validated locally with

```
❯ python -m cudf_polars.experimental.benchmarks.pdsh --path /datasets/toaugspurger/tpch-rs/scale-10/ --suffix "" --iterations 1 18 --validate
❯ python -m cudf_polars.experimental.benchmarks.pdsh --path /datasets/toaugspurger/tpch-rs/scale-10/ --suffix "" --iterations 1 18 --engine duckdb
```

Authors:
  - Tom Augspurger (https://github.com/TomAugspurger)
  - Benjamin Zaitlen (https://github.com/quasiben)
  - Matthew Murray (https://github.com/Matt711)

Approvers:
  - Matthew Murray (https://github.com/Matt711)
  - Richard (Rick) Zamora (https://github.com/rjzamora)

URL: #21119

Author: TomAugspurger

python/cudf_polars/cudf_polars/experimental/benchmarks/pdsh.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2025, NVIDIA CORPORATION & AFFILIATES.
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026, NVIDIA CORPORATION & AFFILIATES.
 # SPDX-License-Identifier: Apache-2.0
 
 """
@@ -648,16 +648,16 @@ def q18(run_config: RunConfig) -> pl.LazyFrame:
             .group_by(
                 "c_name", "o_custkey", "o_orderkey", "o_orderdate", "o_totalprice"
             )
-            .agg(pl.col("l_quantity").sum().alias("col6"))
+            .agg(pl.col("l_quantity").sum().alias("sum(l_quantity)"))
             .select(
                 pl.col("c_name"),
                 pl.col("o_custkey").alias("c_custkey"),
                 pl.col("o_orderkey"),
-                pl.col("o_orderdate").alias("o_orderdat"),
+                pl.col("o_orderdate"),
                 pl.col("o_totalprice"),
-                pl.col("col6"),
+                pl.col("sum(l_quantity)"),
             )
-            .sort(by=["o_totalprice", "o_orderdat"], descending=[True, False])
+            .sort(by=["o_totalprice", "o_orderdate"], descending=[True, False])
             .head(100)
         )
 
@@ -1373,9 +1373,9 @@ def q18(run_config: RunConfig) -> str:
                 c_name,
                 c_custkey,
                 o_orderkey,
-                o_orderdate as o_orderdat,
+                o_orderdate,
                 o_totalprice,
-                sum(l_quantity) as col6
+                sum(l_quantity)
             from
                 customer,
                 orders,