Ensure that lazy frames converted via to-lazy are not converted back to eager frames later in the pipeline. (#12525)

# Description
@maxim-uvarov discovered the following error:
```
> [[a b]; [6 2] [1 4] [4 1]] | polars into-lazy | polars sort-by a | polars unique --subset [a]
Error:   × Error using as series
   ╭─[entry #1:1:68]
 1 │ [[a b]; [6 2] [1 4] [4 1]] | polars into-lazy | polars sort-by a | polars unique --subset [a]
   ·                                                                    ──────┬──────
   ·                                                                          ╰── dataframe has more than one column
   ╰────
 ```
 
During investigation, I discovered the root cause was that the lazy frame was incorrectly converted back to a eager dataframe. In order to keep this from happening, I explicitly set that the dataframe did not come from an eager frame. This causes the conversion logic to not attempt to convert the dataframe later in the pipeline.

---------

Co-authored-by: Jack Wright <jack.wright@disqo.com>

crates/nu_plugin_polars/src/dataframe/lazy/to_lazy.rs

@@ -52,10 +52,33 @@ impl PluginCommand for ToLazyFrame {
             .transpose()?;
 
         let df = NuDataFrame::try_from_iter(plugin, input.into_iter(), maybe_schema)?;
-        let lazy = NuLazyFrame::from_dataframe(df);
+        let mut lazy = NuLazyFrame::from_dataframe(df);
+        // We don't want this converted back to an eager dataframe at some point
+        lazy.from_eager = false;
         Ok(PipelineData::Value(
             lazy.cache(plugin, engine, call.head)?.into_value(call.head),
             None,
         ))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use nu_plugin_test_support::PluginTest;
+    use nu_protocol::{ShellError, Span};
+
+    use super::*;
+
+    #[test]
+    fn test_to_lazy() -> Result<(), ShellError> {
+        let plugin: Arc<PolarsPlugin> = PolarsPlugin::new_test_mode().into();
+        let mut plugin_test = PluginTest::new("polars", Arc::clone(&plugin))?;
+        let pipeline_data = plugin_test.eval("[[a b]; [6 2] [1 4] [4 1]] | polars into-lazy")?;
+        let value = pipeline_data.into_value(Span::test_data());
+        let df = NuLazyFrame::try_from_value(&plugin, &value)?;
+        assert!(!df.from_eager);
+        Ok(())
+    }
+}

crates/nu_plugin_polars/src/dataframe/values/nu_lazyframe/mod.rs

@@ -64,7 +64,7 @@ impl NuLazyFrame {
                 help: None,
                 inner: vec![],
             })
-            .map(|df| NuDataFrame::new(!self.from_eager, df))
+            .map(|df| NuDataFrame::new(false, df))
     }
 
     pub fn apply_with_expr<F>(self, expr: NuExpression, f: F) -> Self

crates/nu_plugin_polars/src/lib.rs

@@ -182,12 +182,18 @@ pub mod test {
     use nu_plugin_test_support::PluginTest;
     use nu_protocol::{ShellError, Span};
 
-    pub fn test_polars_plugin_command(command: &impl PluginCommand) -> Result<(), ShellError> {
-        let plugin = PolarsPlugin {
-            disable_cache_drop: true,
-            ..PolarsPlugin::default()
-        };
+    impl PolarsPlugin {
+        /// Creates a new polars plugin in test mode
+        pub fn new_test_mode() -> Self {
+            PolarsPlugin {
+                disable_cache_drop: true,
+                ..PolarsPlugin::default()
+            }
+        }
+    }
 
+    pub fn test_polars_plugin_command(command: &impl PluginCommand) -> Result<(), ShellError> {
+        let plugin = PolarsPlugin::new_test_mode();
         let examples = command.examples();
 
         // we need to cache values in the examples