Detect automagic-like assignments in notebooks (#9653)

## Summary

Given a statement like `colors = 6`, we currently treat the cell as an
automagic (since `colors` is an automagic) -- i.e., we assume it's
equivalent to `%colors = 6`. This PR adds some additional detection
whereby if the statement is an _assignment_, we avoid treating it as
such. I audited the list of automagics, and I believe this is safe for
all of them.

Closes https://github.com/astral-sh/ruff/issues/8526.

Closes https://github.com/astral-sh/ruff/issues/9648.

## Test Plan

`cargo test`

crates/ruff_linter/src/linter.rs

@@ -795,6 +795,23 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn test_undefined_name() -> Result<(), NotebookError> {
+        let actual = notebook_path("undefined_name.ipynb");
+        let expected = notebook_path("undefined_name.ipynb");
+        let TestedNotebook {
+            messages,
+            source_notebook,
+            ..
+        } = assert_notebook_path(
+            &actual,
+            expected,
+            &settings::LinterSettings::for_rule(Rule::UndefinedName),
+        )?;
+        assert_messages!(messages, actual, source_notebook);
+        Ok(())
+    }
+
     #[test]
     fn test_json_consistency() -> Result<()> {
         let actual_path = notebook_path("before_fix.ipynb");

crates/ruff_linter/src/snapshots/ruff_linter__linter__tests__undefined_name.snap

@@ -0,0 +1,10 @@
+---
+source: crates/ruff_linter/src/linter.rs
+---
+undefined_name.ipynb:cell 3:1:7: F821 Undefined name `undefined`
+  |
+1 | print(undefined)
+  |       ^^^^^^^^^ F821
+  |
+
+

crates/ruff_notebook/resources/test/fixtures/jupyter/cell/automagic_assignment.json

@@ -0,0 +1,8 @@
+{
+  "execution_count": null,
+  "cell_type": "code",
+  "id": "1",
+  "metadata": {},
+  "outputs": [],
+  "source": ["colors = 6\n", "print(colors)"]
+}

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

@@ -0,0 +1,59 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "a0efffbc-85f1-4513-bf49-5387ec3a2a4e",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "colors = 6"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "6e0b2b50-43f2-4f59-951d-9404dd560ae4",
+   "metadata": {
+    "is_executing": true
+   },
+   "outputs": [],
+   "source": [
+    "print(colors)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "outputs": [],
+   "source": [
+    "print(undefined)"
+   ],
+   "metadata": {
+    "collapsed": false
+   },
+   "id": "884e0e4e686fd56"
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python (ruff)",
+   "language": "python",
+   "name": "ruff"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.11.3"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}

crates/ruff_notebook/src/cell.rs

@@ -80,10 +80,11 @@ impl Cell {
         // ```
         //
         // See: https://ipython.readthedocs.io/en/stable/interactive/magics.html
-        if lines
-            .peek()
-            .and_then(|line| line.split_whitespace().next())
-            .is_some_and(|token| {
+        if let Some(line) = lines.peek() {
+            let mut tokens = line.split_whitespace();
+
+            // The first token must be an automagic, like `load_exit`.
+            if tokens.next().is_some_and(|token| {
                 matches!(
                     token,
                     "alias"
@@ -164,10 +165,20 @@ impl Cell {
                         | "xdel"
                         | "xmode"
                 )
-            })
-        {
+            }) {
+                // The second token must _not_ be an operator, like `=` (to avoid false positives).
+                // The assignment operators can never follow an automagic. Some binary operators
+                // _can_, though (e.g., `cd -` is valid), so we omit them.
+                if !tokens.next().is_some_and(|token| {
+                    matches!(
+                        token,
+                        "=" | "+=" | "-=" | "*=" | "/=" | "//=" | "%=" | "**=" | "&=" | "|=" | "^="
+                    )
+                }) {
                     return true;
                 }
+            }
+        }
 
         // Detect cell magics (which operate on multiple lines).
         lines.any(|line| {

crates/ruff_notebook/src/notebook.rs

@@ -454,6 +454,7 @@ mod tests {
     #[test_case("cell_magic", false)]
     #[test_case("valid_cell_magic", true)]
     #[test_case("automagic", false)]
+    #[test_case("automagic_assignment", true)]
     #[test_case("automagics", false)]
     #[test_case("automagic_before_code", false)]
     #[test_case("automagic_after_code", true)]