feat(polars): expand polars unique to allow expressions inputs (#15771)

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# Description
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`polars unique` currently only operates on entire dataframes. This PR
seeks to expand this command to handle expressions as well. See
examples:

```nushell
  Returns unique values in a subset of lazyframe columns
  > [[a]; [2] [1] [2]]
    | polars into-lazy
    | polars select (polars col a | polars unique)
    | polars collect
  ╭───┬───╮
  │ # │ a │
  ├───┼───┤
  │ 0 │ 1 │
  │ 1 │ 2 │
  ╰───┴───╯

  Returns unique values in a subset of lazyframe columns
  > [[a]; [2] [1] [2]]
    | polars into-lazy
    | polars select (polars col a | polars unique --maintain-order)
    | polars collect
  ╭───┬───╮
  │ # │ a │
  ├───┼───┤
  │ 0 │ 2 │
  │ 1 │ 1 │
  ╰───┴───╯
```

# User-Facing Changes
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helps us keep track of breaking changes. -->
No breaking changes. Users have the added option to use `polars unique`
in an expressions context.

# Tests + Formatting
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Example tests have been added to `polars unique`

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/data/unique.rs

@@ -4,7 +4,9 @@ use crate::{
         utils::{extract_sm_strs, extract_strings},
         values::NuLazyFrame,
     },
-    values::{CustomValueSupport, PolarsPluginObject, PolarsPluginType, cant_convert_err},
+    values::{
+        CustomValueSupport, NuExpression, PolarsPluginObject, PolarsPluginType, cant_convert_err,
+    },
 };
 
 use crate::values::{Column, NuDataFrame};
@@ -48,10 +50,16 @@ impl PluginCommand for Unique {
                 "Keep the same order as the original DataFrame (lazy df)",
                 Some('k'),
             )
-            .input_output_type(
-                Type::Custom("dataframe".into()),
-                Type::Custom("dataframe".into()),
-            )
+            .input_output_types(vec![
+                (
+                    Type::Custom("dataframe".into()),
+                    Type::Custom("dataframe".into()),
+                ),
+                (
+                    Type::Custom("expression".into()),
+                    Type::Custom("expression".into()),
+                ),
+            ])
             .category(Category::Custom("dataframe or lazyframe".into()))
     }
 
@@ -123,9 +131,40 @@ impl PluginCommand for Unique {
                 ),
             },
             Example {
-                description: "Creates a is unique expression from a column",
-                example: "col a | unique",
-                result: None,
+                description: "Returns unique values in a subset of lazyframe columns",
+                example: r#"[[a]; [2] [1] [2]]
+    | polars into-lazy
+    | polars select (polars col a | polars unique)
+    | polars collect"#,
+                result: Some(
+                    NuDataFrame::try_from_columns(
+                        vec![Column::new(
+                            "a".to_string(),
+                            vec![Value::test_int(1), Value::test_int(2)],
+                        )],
+                        None,
+                    )
+                    .expect("simple df for test should not fail")
+                    .into_value(Span::test_data()),
+                ),
+            },
+            Example {
+                description: "Returns unique values in a subset of lazyframe columns",
+                example: r#"[[a]; [2] [1] [2]]
+    | polars into-lazy
+    | polars select (polars col a | polars unique --maintain-order)
+    | polars collect"#,
+                result: Some(
+                    NuDataFrame::try_from_columns(
+                        vec![Column::new(
+                            "a".to_string(),
+                            vec![Value::test_int(2), Value::test_int(1)],
+                        )],
+                        None,
+                    )
+                    .expect("simple df for test should not fail")
+                    .into_value(Span::test_data()),
+                ),
             },
         ]
     }
@@ -142,11 +181,21 @@ impl PluginCommand for Unique {
         match PolarsPluginObject::try_from_value(plugin, &value)? {
             PolarsPluginObject::NuDataFrame(df) => command_eager(plugin, engine, call, df),
             PolarsPluginObject::NuLazyFrame(lazy) => command_lazy(plugin, engine, call, lazy),
+            PolarsPluginObject::NuExpression(expr) => {
+                let maintain = call.has_flag("maintain-order")?;
+                let res: NuExpression = if maintain {
+                    expr.into_polars().unique_stable().into()
+                } else {
+                    expr.into_polars().unique().into()
+                };
+                res.to_pipeline_data(plugin, engine, call.head)
+            }
             _ => Err(cant_convert_err(
                 &value,
                 &[
                     PolarsPluginType::NuDataFrame,
                     PolarsPluginType::NuLazyGroupBy,
+                    PolarsPluginType::NuExpression,
                 ],
             )),
         }