Add roundtrip test case, use simple HashMap

crates/ruff_notebook/resources/test/fixtures/jupyter/vscode_language_id.ipynb

@@ -13,6 +13,11 @@
    "cell_type": "code",
    "execution_count": null,
    "metadata": {
+    "collapsed": true,
+    "jupyter": {
+     "outputs_hidden": true,
+     "source_hidden": true
+    },
     "vscode": {
      "languageId": "javascript"
     }

crates/ruff_notebook/resources/test/fixtures/jupyter/vscode_language_id_expected.ipynb

@@ -13,6 +13,11 @@
    "cell_type": "code",
    "execution_count": null,
    "metadata": {
+    "collapsed": true,
+    "jupyter": {
+     "outputs_hidden": true,
+     "source_hidden": true
+    },
     "vscode": {
      "languageId": "javascript"
     }

crates/ruff_notebook/src/notebook.rs

@@ -596,4 +596,12 @@ print("after empty cells")
         );
         Ok(())
     }
+
+    #[test]
+    fn round_trip() {
+        let path = notebook_path("vscode_language_id.ipynb");
+        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

@@ -18,7 +18,7 @@
 //!   a code cell or not without looking at the `cell_type` property, which
 //!   would require a custom serializer.
 
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, HashMap};
 
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
@@ -171,7 +171,7 @@ pub struct CellMetadata {
     pub vscode: Option<CodeCellMetadataVSCode>,
     /// Catch-all for metadata that isn't required by Ruff.
     #[serde(flatten)]
-    pub extra: BTreeMap<String, Value>,
+    pub extra: HashMap<String, Value>,
 }
 
 /// VS Code specific cell metadata.