fix(polars): cast as date now returns Date type instead of Datetime<ns> (#15574)

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# Description
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This PR fixes the bug where various commands that cast a column as a
`date` type would return `datetime<ns>` rather than the intended type
`date`. Affected commands include `polars into-df --schema`, `polars
into-lazy --schema`, `polars as-date`, and `polars cast date`.

This bug derives from the fact that Nushell uses the `date` type to
denote a datetime type whereas polars differentiates between `Date` and
`Datetime` types. By default, this PR retains the behavior that a
Nushell `date` type will be mapped to a polars `Datetime<ns>` unless
otherwise specified.

```nushell
#  Current (erroneous) implementation
> [[a]; [2025-03-20]] | polars into-df --schema {a: "date"} | polars schema
╭───┬──────────────╮
│ a │ datetime<ns> │
╰───┴──────────────╯

#  Fixed implementation
> [[a]; [2025-03-20]] | polars into-df --schema {a: "date"} | polars schema
╭───┬──────╮
│ a │ date │
╰───┴──────╯

#  Fixed implementation: by default, Nushell dates map to datetime<ns>
> [[a]; [2025-03-20]] | polars into-df | polars schema
╭───┬───────────────────╮
│ a │ datetime<ns, UTC> │
╰───┴───────────────────╯
```

# User-Facing Changes
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Soft breaking change: users previously who wanted to cast a date column
to type `date` can now expect the output to be type `date` instead of
`datetime<ns>`.

# Tests + Formatting
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Example test added to `polars as-date` command.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/datetime/as_date.rs

@@ -4,7 +4,8 @@ use super::super::super::values::NuDataFrame;
 
 use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
 use nu_protocol::{
-    Category, Example, LabeledError, PipelineData, ShellError, Signature, SyntaxShape, Type,
+    record, Category, Example, LabeledError, PipelineData, ShellError, Signature, Span,
+    SyntaxShape, Type, Value,
 };
 use polars::prelude::{IntoSeries, StringMethods};
 
@@ -52,6 +53,16 @@ impl PluginCommand for AsDate {
                 example: r#"["2021-12-30" "2021-12-31 21:00:00"] | polars into-df | polars as-date "%Y-%m-%d" --not-exact"#,
                 result: None,
             },
+            Example {
+                description: "Output is of date type",
+                example: r#"["2021-12-30" "2021-12-31 21:00:00"] | polars into-df | polars as-date "%Y-%m-%d" --not-exact | polars schema"#,
+                result: Some(Value::record(
+                    record! {
+                        "date" => Value::string("date", Span::test_data()),
+                    },
+                    Span::test_data(),
+                )),
+            },
         ]
     }
 

crates/nu_plugin_polars/src/dataframe/values/nu_dataframe/conversion.rs

@@ -12,8 +12,8 @@ use polars::datatypes::{AnyValue, PlSmallStr};
 use polars::prelude::{
     ChunkAnyValue, Column as PolarsColumn, DataFrame, DataType, DatetimeChunked, Float32Type,
     Float64Type, Int16Type, Int32Type, Int64Type, Int8Type, IntoSeries, ListBooleanChunkedBuilder,
-    ListBuilderTrait, ListPrimitiveChunkedBuilder, ListStringChunkedBuilder, ListType, NamedFrom,
-    NewChunkedArray, ObjectType, PolarsError, Schema, SchemaExt, Series, StructChunked,
+    ListBuilderTrait, ListPrimitiveChunkedBuilder, ListStringChunkedBuilder, ListType, LogicalType,
+    NamedFrom, NewChunkedArray, ObjectType, PolarsError, Schema, SchemaExt, Series, StructChunked,
     TemporalMethods, TimeUnit, UInt16Type, UInt32Type, UInt64Type, UInt8Type,
 };
 
@@ -441,10 +441,16 @@ fn typed_column_to_series(name: PlSmallStr, column: TypedColumn) -> Result<Serie
                 }
             });
 
-            let res: DatetimeChunked = ChunkedArray::<Int64Type>::from_iter_options(name, it)
-                .into_datetime(TimeUnit::Nanoseconds, None);
-
-            Ok(res.into_series())
+            ChunkedArray::<Int64Type>::from_iter_options(name, it)
+                .into_datetime(TimeUnit::Nanoseconds, None)
+                .cast_with_options(&DataType::Date, Default::default())
+                .map_err(|e| ShellError::GenericError {
+                    error: "Error parsing date".into(),
+                    msg: "".into(),
+                    span: None,
+                    help: Some(e.to_string()),
+                    inner: vec![],
+                })
         }
         DataType::Datetime(tu, maybe_tz) => {
             let dates = column