Add a ScopeKind for the __class__ cell (#20048)

Summary
--

This PR aims to resolve (or help to resolve) #18442 and #19357 by
encoding the CPython semantics around the `__class__` cell in our
semantic model. Namely,

> `__class__` is an implicit closure reference created by the compiler
if any methods in a class body refer to either `__class__` or super.

from the Python
[docs](https://docs.python.org/3/reference/datamodel.html#creating-the-class-object).

As noted in the variant docs by @AlexWaygood, we don't fully model this
behavior, opting always to create the `__class__` cell binding in a new
`ScopeKind::DunderClassCell` around each method definition, without
checking if any method in the class body actually refers to `__class__`
or `super`.

As such, this PR fixes #18442 but not #19357.

Test Plan
--

Existing tests, plus the tests from #19783, which now pass without any
rule-specific code.

Note that we opted not to alter the behavior of F841 here because
flagging `__class__` in these cases still seems helpful. See the
discussion in
https://github.com/astral-sh/ruff/pull/20048#discussion_r2296252395 and
in the test comments for more information.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Mikko Leppänen <mleppan23@gmail.com>

crates/ruff_python_semantic/src/binding.rs

@@ -446,7 +446,7 @@ impl Ranged for Binding<'_> {
 /// ID uniquely identifying a [Binding] in a program.
 ///
 /// Using a `u32` to identify [Binding]s should be sufficient because Ruff only supports documents with a
-/// size smaller than or equal to `u32::max`. A document with the size of `u32::max` must have fewer than `u32::max`
+/// size smaller than or equal to `u32::MAX`. A document with the size of `u32::MAX` must have fewer than `u32::MAX`
 /// bindings because bindings must be separated by whitespace (and have an assignment).
 #[newtype_index]
 pub struct BindingId;
@@ -672,6 +672,24 @@ pub enum BindingKind<'a> {
     /// Stores the ID of the binding that was shadowed in the enclosing
     /// scope, if any.
     UnboundException(Option<BindingId>),
+
+    /// A binding to `__class__` in the implicit closure created around every method in a class
+    /// body, if any method refers to either `__class__` or `super`.
+    ///
+    /// ```python
+    /// class C:
+    ///     __class__  # NameError: name '__class__' is not defined
+    ///
+    ///     def f():
+    ///         print(__class__)  # allowed
+    ///
+    ///     def g():
+    ///         nonlocal __class__  # also allowed because the scope is *not* the function scope
+    /// ```
+    ///
+    /// See <https://docs.python.org/3/reference/datamodel.html#creating-the-class-object> for more
+    /// details.
+    DunderClassCell,
 }
 
 bitflags! {