[`pyflakes`] Fix false positive for `__class__` in lambda expressions within class definitions (`F821`) (#20564)

## Summary

Fixes #20562

---------

Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_33.py

@@ -0,0 +1,21 @@
+class C:
+    f = lambda self: __class__
+
+
+print(C().f().__name__)
+
+# Test: nested lambda
+class D:
+    g = lambda self: (lambda: __class__)
+
+
+print(D().g()().__name__)
+
+# Test: lambda outside class (should still fail)
+h = lambda: __class__
+
+# Test: lambda referencing module-level variable (should not be flagged as F821)
+import uuid
+
+class E:
+    uuid = lambda: str(uuid.uuid4())

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -2116,7 +2116,7 @@ impl<'a> Visitor<'a> for Checker<'a> {
             | Expr::DictComp(_)
             | Expr::SetComp(_) => {
                 self.analyze.scopes.push(self.semantic.scope_id);
-                self.semantic.pop_scope();
+                self.semantic.pop_scope(); // Lambda/Generator/Comprehension scope
             }
             _ => {}
         }
@@ -3041,7 +3041,35 @@ impl<'a> Checker<'a> {
                 if let Some(parameters) = parameters {
                     self.visit_parameters(parameters);
                 }
+
+                // Here we add the implicit scope surrounding a lambda which allows code in the
+                // lambda to access `__class__` at runtime when the lambda is defined within a class.
+                // See the `ScopeKind::DunderClassCell` docs for more information.
+                let added_dunder_class_scope = if self
+                    .semantic
+                    .current_scopes()
+                    .any(|scope| scope.kind.is_class())
+                {
+                    self.semantic.push_scope(ScopeKind::DunderClassCell);
+                    let binding_id = self.semantic.push_binding(
+                        TextRange::default(),
+                        BindingKind::DunderClassCell,
+                        BindingFlags::empty(),
+                    );
+                    self.semantic
+                        .current_scope_mut()
+                        .add("__class__", binding_id);
+                    true
+                } else {
+                    false
+                };
+
                 self.visit_expr(body);
+
+                // Pop the DunderClassCell scope if it was added
+                if added_dunder_class_scope {
+                    self.semantic.pop_scope();
+                }
             }
         }
         self.semantic.restore(snapshot);

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -166,6 +166,7 @@ mod tests {
     #[test_case(Rule::UndefinedName, Path::new("F821_30.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_31.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_32.pyi"))]
+    #[test_case(Rule::UndefinedName, Path::new("F821_33.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_0.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_0.pyi"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_1.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F821_F821_33.py.snap

@@ -0,0 +1,12 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F821 Undefined name `__class__`
+  --> F821_33.py:15:13
+   |
+14 | # Test: lambda outside class (should still fail)
+15 | h = lambda: __class__
+   |             ^^^^^^^^^
+16 |
+17 | # Test: lambda referencing module-level variable (should not be flagged as F821)
+   |