feat(polars): enable parsing strings as dates and datetime in polars schema (#15645)

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# Description
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This PR seeks to add a quality-of-life feature that enables date and
datetime parsing of strings in `polars into-df`, `polars into-lazy`, and
`polars open`, and avoid the more verbose method of casting each column
into date/datetime. Currently, setting the schema to `date` on a `str`
column would silently error as a null column. See a comparison of the
current and proposed implementations.

The proposed implementation assumes a date format "%Y-%m-%d" and a
datetime format of "%Y-%m-%d %H:%M:%S" for naive datetimes and "%Y-%m-%d
%H:%M:%S%:z" for timezone-aware datetimes. Other formats must be
specified via parsing through `polars as-date` and `polars as-datetime`.

```nushell
#  Current Implementations
> [[a]; ["2025-04-01"]] | polars into-df --schema {a: date}
╭───┬───╮
│ # │ a │
├───┼───┤
│ 0 │   │
╰───┴───╯

> [[a]; ["2025-04-01 01:00:00"]] | polars into-df --schema {a: "datetime<ns,*>"}
╭───┬───╮
│ # │ a │
├───┼───┤
│ 0 │   │
╰───┴───╯

#  Proposed Implementation
> [[a]; ["2025-04-01"]] | polars into-df --schema {a: date}
╭───┬─────────────────────╮
│ # │          a          │
├───┼─────────────────────┤
│ 0 │ 04/01/25 12:00:00AM │
╰───┴─────────────────────╯

> [[a]; ["2025-04-01 01:00:00"]] | polars into-df --schema {a: "datetime<ns,*>"}
╭───┬─────────────────────╮
│ # │          a          │
├───┼─────────────────────┤
│ 0 │ 04/01/25 01:00:00AM │
╰───┴─────────────────────╯

> [[a]; ["2025-04-01 01:00:00-04:00"]] | polars into-df --schema {a: "datetime<ns,UTC>"}
╭───┬─────────────────────╮
│ # │          a          │
├───┼─────────────────────┤
│ 0 │ 04/01/25 05:00:00AM │
╰───┴─────────────────────╯
```

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
No breaking changes. Users have the added option to parse string columns
into date/datetimes.

# Tests + Formatting
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No tests were added to any examples.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/values/nu_dataframe/conversion.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 use std::ops::{Deref, DerefMut};
 use std::sync::Arc;
 
-use chrono::{DateTime, Duration, FixedOffset, NaiveTime, TimeZone, Utc};
+use chrono::{DateTime, Duration, FixedOffset, NaiveDate, NaiveDateTime, NaiveTime, TimeZone, Utc};
 use chrono_tz::Tz;
 use indexmap::map::{Entry, IndexMap};
 use polars::chunked_array::builder::AnonymousOwnedListBuilder;
@@ -461,15 +461,34 @@ fn typed_column_to_series(name: PlSmallStr, column: TypedColumn) -> Result<Serie
             }
         }
         DataType::Date => {
-            let it = column.values.iter().map(|v| {
-                if let Value::Date { val, .. } = &v {
-                    Some(val.timestamp_nanos_opt().unwrap_or_default())
-                } else {
-                    None
-                }
-            });
+            let it = column
+                .values
+                .iter()
+                .map(|v| match &v {
+                    Value::Date { val, .. } => {
+                        Ok(Some(val.timestamp_nanos_opt().unwrap_or_default()))
+                    }
 
-            ChunkedArray::<Int64Type>::from_iter_options(name, it)
+                    Value::String { val, .. } => {
+                        let expected_format = "%Y-%m-%d";
+                        let nanos = NaiveDate::parse_from_str(val, expected_format)
+                            .map_err(|e| ShellError::GenericError {
+                                error: format!("Error parsing date from string: {e}"),
+                                msg: "".into(),
+                                span: None,
+                                help: Some(format!("Expected format {expected_format}. If you need to parse with another format, please set the schema to `str` and parse with `polars as-date <format>`.")),
+                                inner: vec![],
+                            })?
+                            .and_hms_nano_opt(0, 0, 0, 0)
+                            .and_then(|dt| dt.and_utc().timestamp_nanos_opt());
+                        Ok(nanos)
+                    }
+
+                    _ => Ok(None),
+                })
+                .collect::<Result<Vec<_>, ShellError>>()?;
+
+            ChunkedArray::<Int64Type>::from_iter_options(name, it.into_iter())
                 .into_datetime(TimeUnit::Nanoseconds, None)
                 .cast_with_options(&DataType::Date, Default::default())
                 .map_err(|e| ShellError::GenericError {
@@ -507,6 +526,39 @@ fn typed_column_to_series(name: PlSmallStr, column: TypedColumn) -> Result<Serie
                             .map(|nanos| nanos_to_timeunit(nanos, *tu))
                             .transpose(),
 
+                        (Some(_), Value::String { val, .. }) => {
+                            // because we're converting to the number of nano seconds since epoch, the timezone is irrelevant
+                            let expected_format = "%Y-%m-%d %H:%M:%S%:z";
+                            DateTime::parse_from_str(val, expected_format)
+                                .map_err(|e| ShellError::GenericError {
+                                    error: format!("Error parsing datetime from string: {e}"),
+                                    msg: "".into(),
+                                    span: None,
+                                    help: Some(format!("Expected format {expected_format}. If you need to parse with another format, please set the schema to `str` and parse with `polars as-datetime <format>`.")),
+                                    inner: vec![],
+                                })?
+                                .timestamp_nanos_opt()
+                                .map(|nanos| nanos_to_timeunit(nanos, *tu))
+                                .transpose()
+                        }
+
+                        (None, Value::String { val, .. }) => {
+                            let expected_format = "%Y-%m-%d %H:%M:%S";
+
+                            NaiveDateTime::parse_from_str(val, expected_format)
+                                .map_err(|e| ShellError::GenericError {
+                                    error: format!("Error parsing datetime from string: {e}"),
+                                    msg: "".into(),
+                                    span: None,
+                                    help: Some(format!("Expected format {expected_format}. If you need to parse with another format, please set the schema to `str` and parse with `polars as-datetime <format>`.")),
+                                    inner: vec![],
+                                })?
+                                .and_utc()
+                                .timestamp_nanos_opt()
+                                .map(|nanos| nanos_to_timeunit(nanos, *tu))
+                                .transpose()
+                        }
+
                         _ => Ok(None),
                     }
                 })