Allow transparent cell magics (#8911)

## Summary

This PR updates the logic for `is_magic_cell` to include certain cell
magics. These cell magics would contain Python code following the line
defining the command. The code could define a variable which can then be
referenced in other cells. Currently, we would ignore the cell
completely leading to undefined-name violation.

As discussed in
https://github.com/astral-sh/ruff/issues/8354#issuecomment-1832221009

## Test Plan

Add new test case to validate this scenario.

crates/ruff_notebook/src/cell.rs

@@ -170,7 +170,50 @@ impl Cell {
         }
 
         // Detect cell magics (which operate on multiple lines).
-        lines.any(|line| line.trim_start().starts_with("%%"))
+        lines.any(|line| {
+            line.split_whitespace().next().is_some_and(|first| {
+                if first.len() < 2 {
+                    return false;
+                }
+                let (token, command) = first.split_at(2);
+                // These cell magics are special in that the lines following them are valid
+                // Python code and the variables defined in that scope are available to the
+                // rest of the notebook.
+                //
+                // For example:
+                //
+                // Cell 1:
+                // ```python
+                // x = 1
+                // ```
+                //
+                // Cell 2:
+                // ```python
+                // %%time
+                // y = x
+                // ```
+                //
+                // Cell 3:
+                // ```python
+                // print(y)  # Here, `y` is available.
+                // ```
+                //
+                // This is to avoid false positives when these variables are referenced
+                // elsewhere in the notebook.
+                token == "%%"
+                    && !matches!(
+                        command,
+                        "capture"
+                            | "debug"
+                            | "prun"
+                            | "pypy"
+                            | "python"
+                            | "python3"
+                            | "time"
+                            | "timeit"
+                    )
+            })
+        })
     }
 }
 

crates/ruff_notebook/src/notebook.rs

@@ -426,6 +426,7 @@ mod tests {
     #[test_case(Path::new("code_and_magic.json"), true; "code_and_magic")]
     #[test_case(Path::new("only_code.json"), true; "only_code")]
     #[test_case(Path::new("cell_magic.json"), false; "cell_magic")]
+    #[test_case(Path::new("valid_cell_magic.json"), true; "valid_cell_magic")]
     #[test_case(Path::new("automagic.json"), false; "automagic")]
     #[test_case(Path::new("automagics.json"), false; "automagics")]
     #[test_case(Path::new("automagic_before_code.json"), false; "automagic_before_code")]