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");