feat(polars): loosen constraints on accepted expressions in polars group-by (#15583)

# Description
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This PR lifts the constraint that expressions in the `polars group-by`
command must be limited only to the type `Expr::Column` rather than most
`Expr` types, which is what the underlying polars crate allows. This
change enables more complex expressions to group by.

In the example below, we group by even or odd days of column `a`. While
we can reach the same result by creating and grouping by a new column in
two separate steps, integrating these steps in a single group-by allows
for better delegation to the polars optimizer.

```nushell
#  Group by an expression and perform an aggregation
  > [[a b]; [2025-04-01 1] [2025-04-02 2] [2025-04-03 3] [2025-04-04 4]]
    | polars into-lazy
    | polars group-by (polars col a | polars get-day | $in mod 2)
    | polars agg [
        (polars col b | polars min | polars as "b_min")
        (polars col b | polars max | polars as "b_max")
        (polars col b | polars sum | polars as "b_sum")
     ]
    | polars collect
    | polars sort-by a
  ╭───┬───┬───────┬───────┬───────╮
  │ # │ a │ b_min │ b_max │ b_sum │
  ├───┼───┼───────┼───────┼───────┤
  │ 0 │ 0 │     2 │     4 │     6 │
  │ 1 │ 1 │     1 │     3 │     4 │
  ╰───┴───┴───────┴───────┴───────╯
```  

# User-Facing Changes
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helps us keep track of breaking changes. -->
No breaking changes. The user is empowered to use more complex
expressions in `polars group-by`

# Tests + Formatting
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An example is added to `polars group-by`.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/aggregation/groupby.rs

@@ -39,7 +39,8 @@ impl PluginCommand for ToLazyGroupBy {
     }
 
     fn examples(&self) -> Vec<Example> {
-        vec![Example {
+        vec![
+            Example {
                 description: "Group by and perform an aggregation",
                 example: r#"[[a b]; [1 2] [1 4] [2 6] [2 4]]
     | polars into-lazy
@@ -63,7 +64,33 @@ impl PluginCommand for ToLazyGroupBy {
                     )
                     .into_value(Span::test_data()),
                 ),
-        }]
+            },
+            Example {
+                description: "Group by an expression and perform an aggregation",
+                example: r#"[[a b]; [2025-04-01 1] [2025-04-02 2] [2025-04-03 3] [2025-04-04 4]]
+    | polars into-lazy
+    | polars group-by (polars col a | polars get-day | $in mod 2)
+    | polars agg [
+        (polars col b | polars min | polars as "b_min")
+        (polars col b | polars max | polars as "b_max")
+        (polars col b | polars sum | polars as "b_sum")
+     ]
+    | polars collect
+    | polars sort-by a"#,
+                result: Some(
+                    NuDataFrame::from(
+                        df!(
+                            "a" => &[0i64, 1],
+                            "b_min" => &[2i64, 1],
+                            "b_max" => &[4i64, 3],
+                            "b_sum" => &[6i64, 4],
+                        )
+                        .expect("should not fail"),
+                    )
+                    .into_value(Span::test_data()),
+                ),
+            },
+        ]
     }
 
     fn run(
@@ -79,11 +106,11 @@ impl PluginCommand for ToLazyGroupBy {
 
         if expressions
             .iter()
-            .any(|expr| !matches!(expr, Expr::Column(..)))
+            .any(|expr| matches!(expr, Expr::Agg(..) | Expr::Window { .. }))
         {
             let value: Value = call.req(0)?;
             Err(ShellError::IncompatibleParametersSingle {
-                msg: "Expected only Col expressions".into(),
+                msg: "Cannot group by an aggregation or window expression".into(),
                 span: value.span(),
             })?;
         }