Fallback to kernelspec to check if it's a Python notebook (#12875)

## Summary

This PR adds a fallback logic for `is_python_notebook` to check the
`kernelspec.language` field.

Reference implementation in VS Code:
1c31e75898/extensions/ipynb/src/deserializers.ts (L20-L22)

It's also required for the kernel to provide the `language` they're
implementing based on
https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs
reference although that's for the `kernel.json` file but is also
included in the notebook metadata.

Closes: #12281

## Test Plan

Add a test case for `is_python_notebook` and include the test notebook
for round trip validation.

The test notebook contains two cells, one is JavaScript (denoted via the
`vscode.languageId` metadata) and the other is Python (no metadata). The
notebook metadata only contains `kernelspec` and the `language_info` is
absent.

I also verified that this is a valid notebook by opening it in Jupyter
Lab, VS Code and using `nbformat` validator.

crates/ruff_notebook/src/notebook.rs

@@ -408,13 +408,18 @@ impl Notebook {
         &self.raw.metadata
     }
 
-    /// Return `true` if the notebook is a Python notebook, `false` otherwise.
+    /// Check if it's a Python notebook.
+    ///
+    /// This is determined by checking the `language_info` or `kernelspec` in the notebook
+    /// metadata. If neither is present, it's assumed to be a Python notebook.
     pub fn is_python_notebook(&self) -> bool {
-        self.raw
-            .metadata
-            .language_info
-            .as_ref()
-            .map_or(true, |language| language.name == "python")
+        if let Some(language_info) = self.raw.metadata.language_info.as_ref() {
+            return language_info.name == "python";
+        }
+        if let Some(kernel_spec) = self.raw.metadata.kernelspec.as_ref() {
+            return kernel_spec.language.as_deref() == Some("python");
+        }
+        true
     }
 
     /// Write the notebook back to the given [`Write`] implementer.
@@ -456,18 +461,12 @@ mod tests {
         Path::new("./resources/test/fixtures/jupyter").join(path)
     }
 
-    #[test]
-    fn test_python() -> Result<(), NotebookError> {
-        let notebook = Notebook::from_path(&notebook_path("valid.ipynb"))?;
-        assert!(notebook.is_python_notebook());
-        Ok(())
-    }
-
-    #[test]
-    fn test_r() -> Result<(), NotebookError> {
-        let notebook = Notebook::from_path(&notebook_path("R.ipynb"))?;
-        assert!(!notebook.is_python_notebook());
-        Ok(())
+    #[test_case("valid.ipynb", true)]
+    #[test_case("R.ipynb", false)]
+    #[test_case("kernelspec_language.ipynb", true)]
+    fn is_python_notebook(filename: &str, expected: bool) {
+        let notebook = Notebook::from_path(&notebook_path(filename)).unwrap();
+        assert_eq!(notebook.is_python_notebook(), expected);
     }
 
     #[test]
@@ -597,9 +596,10 @@ print("after empty cells")
         Ok(())
     }
 
-    #[test]
-    fn round_trip() {
-        let path = notebook_path("vscode_language_id.ipynb");
+    #[test_case("vscode_language_id.ipynb")]
+    #[test_case("kernelspec_language.ipynb")]
+    fn round_trip(filename: &str) {
+        let path = notebook_path(filename);
         let expected = std::fs::read_to_string(&path).unwrap();
         let actual = super::round_trip(&path).unwrap();
         assert_eq!(actual, expected);

crates/ruff_notebook/src/schema.rs

@@ -169,7 +169,7 @@ pub struct CellMetadata {
     /// preferred language.
     /// <https://github.com/microsoft/vscode/blob/e6c009a3d4ee60f352212b978934f52c4689fbd9/extensions/ipynb/src/serializers.ts#L117-L122>
     pub vscode: Option<CodeCellMetadataVSCode>,
-    /// Catch-all for metadata that isn't required by Ruff.
+    /// For additional properties that isn't required by Ruff.
     #[serde(flatten)]
     pub extra: HashMap<String, Value>,
 }
@@ -190,8 +190,8 @@ pub struct RawNotebookMetadata {
     /// The author(s) of the notebook document
     pub authors: Option<Value>,
     /// Kernel information.
-    pub kernelspec: Option<Value>,
-    /// Kernel information.
+    pub kernelspec: Option<Kernelspec>,
+    /// Language information.
     pub language_info: Option<LanguageInfo>,
     /// Original notebook format (major number) before converting the notebook between versions.
     /// This should never be written to a file.
@@ -206,6 +206,23 @@ pub struct RawNotebookMetadata {
 /// Kernel information.
 #[skip_serializing_none]
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Kernelspec {
+    /// The language name. This isn't mentioned in the spec but is populated by various tools and
+    /// can be used as a fallback if [`language_info`] is missing.
+    ///
+    /// This is also used by VS Code to determine the preferred language of the notebook:
+    /// <https://github.com/microsoft/vscode/blob/1c31e758985efe11bc0453a45ea0bb6887e670a4/extensions/ipynb/src/deserializers.ts#L20-L22>.
+    ///
+    /// [`language_info`]: RawNotebookMetadata::language_info
+    pub language: Option<String>,
+    /// For additional properties that isn't required by Ruff.
+    #[serde(flatten)]
+    pub extra: HashMap<String, Value>,
+}
+
+/// Language information.
+#[skip_serializing_none]
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 pub struct LanguageInfo {
     /// The codemirror mode to use for code in this language.
     pub codemirror_mode: Option<Value>,