Add tests of implemented StringFunctions (#16007)

Additionally, assert that we raise during translation for an unhandled function.

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - https://github.com/brandon-b-miller

URL: #16007

python/cudf_polars/cudf_polars/dsl/expr.py

@@ -691,7 +691,9 @@ def do_evaluate(
                 )
             )
         else:
-            raise NotImplementedError(f"StringFunction {self.name}")
+            raise NotImplementedError(
+                f"StringFunction {self.name}"
+            )  # pragma: no cover; handled by init raising
 
 
 class Sort(Expr):

python/cudf_polars/tests/expressions/test_stringfunction.py

@@ -0,0 +1,41 @@
+# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+# SPDX-License-Identifier: Apache-2.0
+from __future__ import annotations
+
+import pytest
+
+import polars as pl
+
+from cudf_polars import translate_ir
+from cudf_polars.testing.asserts import assert_gpu_result_equal
+
+
+def test_supported_stringfunction_expression():
+    ldf = pl.LazyFrame(
+        {
+            "a": ["a", "b", "cdefg", "h", "Wıth ünιcοde"],  # noqa: RUF001
+            "b": [0, 3, 1, -1, None],
+        }
+    )
+
+    query = ldf.select(
+        pl.col("a").str.starts_with("Z"),
+        pl.col("a").str.ends_with("h").alias("endswith_h"),
+        pl.col("a").str.to_lowercase().alias("lower"),
+        pl.col("a").str.to_uppercase().alias("upper"),
+    )
+    assert_gpu_result_equal(query)
+
+
+def test_unsupported_stringfunction():
+    ldf = pl.LazyFrame(
+        {
+            "a": ["a", "b", "cdefg", "h", "Wıth ünιcοde"],  # noqa: RUF001
+            "b": [0, 3, 1, -1, None],
+        }
+    )
+
+    q = ldf.select(pl.col("a").str.count_matches("e", literal=True))
+
+    with pytest.raises(NotImplementedError):
+        _ = translate_ir(q._ldf.visit())