fix(rust, python): don't panic on ignored context (#5958)

Author: ritchie46

py-polars/tests/unit/test_context.py

@@ -0,0 +1,16 @@
+import polars as pl
+
+
+def test_context_ignore_5867() -> None:
+    outer = pl.DataFrame({"OtherCol": [1, 2, 3, 4]}).lazy()
+    df = (
+        pl.DataFrame({"Category": [1, 1, 2, 2], "Counts": [1, 2, 3, 4]})
+        .lazy()
+        .with_context(outer)
+    )
+    assert (
+        df.groupby("Category", maintain_order=True)
+        .agg([(pl.col("Counts")).sum()])
+        .collect()
+        .to_dict(False)
+    ) == {"Category": [1, 2], "Counts": [3, 7]}