[ty] Improve subscript narrowing for "safe mutable classes" (#19781)

## Summary

This PR improves the `is_safe_mutable_class` function in `infer.rs` in
several ways:
- It uses `KnownClass::to_instance()` for all "safe mutable classes".
Previously, we were using `SpecialFormType::instance_fallback()` for
some variants -- I'm not totally sure why. Switching to
`KnownClass::to_instance()` for all "safe mutable classes" fixes a
number of TODOs in the `assignment.md` mdtest suite
- Rather than eagerly calling `.to_instance(db)` on all "safe mutable
classes" every time `is_safe_mutable_class` is called, we now only call
it lazily on each element, allowing us to short-circuit more
effectively.
- I removed the entry entirely for `TypedDict` from the list of "safe
mutable classes", as it's not correct.
`SpecialFormType::TypedDict.instance_fallback(db)` just returns an
instance type representing "any instance of `typing._SpecialForm`",
which I don't think was the intent of this code. No tests fail as a
result of removing this entry, as we already check separately whether an
object is an inhabitant of a `TypedDict` type (and consider that object
safe-mutable if so!).

## Test Plan

mdtests updated

crates/ty_python_semantic/src/types/infer.rs

@@ -1785,6 +1785,39 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
     }
 
     fn add_binding(&mut self, node: AnyNodeRef, binding: Definition<'db>, ty: Type<'db>) {
+        /// Arbitrary `__getitem__`/`__setitem__` methods on a class do not
+        /// necessarily guarantee that the passed-in value for `__setitem__` is stored and
+        /// can be retrieved unmodified via `__getitem__`. Therefore, we currently only
+        /// perform assignment-based narrowing on a few built-in classes (`list`, `dict`,
+        /// `bytesarray`, `TypedDict` and `collections` types) where we are confident that
+        /// this kind of narrowing can be performed soundly. This is the same approach as
+        /// pyright. TODO: Other standard library classes may also be considered safe. Also,
+        /// subclasses of these safe classes that do not override `__getitem__/__setitem__`
+        /// may be considered safe.
+        fn is_safe_mutable_class<'db>(db: &'db dyn Db, ty: Type<'db>) -> bool {
+            const SAFE_MUTABLE_CLASSES: &[KnownClass] = &[
+                KnownClass::List,
+                KnownClass::Dict,
+                KnownClass::Bytearray,
+                KnownClass::DefaultDict,
+                KnownClass::ChainMap,
+                KnownClass::Counter,
+                KnownClass::Deque,
+                KnownClass::OrderedDict,
+            ];
+
+            SAFE_MUTABLE_CLASSES
+                .iter()
+                .map(|class| class.to_instance(db))
+                .any(|safe_mutable_class| {
+                    ty.is_equivalent_to(db, safe_mutable_class)
+                        || ty
+                            .generic_origin(db)
+                            .zip(safe_mutable_class.generic_origin(db))
+                            .is_some_and(|(l, r)| l == r)
+                })
+        }
+
         debug_assert!(
             binding
                 .kind(self.db())
@@ -2026,38 +2059,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             let value_ty = self
                 .try_expression_type(value)
                 .unwrap_or_else(|| self.infer_expression(value));
-            // Arbitrary `__getitem__`/`__setitem__` methods on a class do not
-            // necessarily guarantee that the passed-in value for `__setitem__` is stored and
-            // can be retrieved unmodified via `__getitem__`. Therefore, we currently only
-            // perform assignment-based narrowing on a few built-in classes (`list`, `dict`,
-            // `bytesarray`, `TypedDict` and `collections` types) where we are confident that
-            // this kind of narrowing can be performed soundly. This is the same approach as
-            // pyright. TODO: Other standard library classes may also be considered safe. Also,
-            // subclasses of these safe classes that do not override `__getitem__/__setitem__`
-            // may be considered safe.
-            let is_safe_mutable_class = || {
-                let safe_mutable_classes = [
-                    KnownClass::List.to_instance(db),
-                    KnownClass::Dict.to_instance(db),
-                    KnownClass::Bytearray.to_instance(db),
-                    KnownClass::DefaultDict.to_instance(db),
-                    SpecialFormType::ChainMap.instance_fallback(db),
-                    SpecialFormType::Counter.instance_fallback(db),
-                    SpecialFormType::Deque.instance_fallback(db),
-                    SpecialFormType::OrderedDict.instance_fallback(db),
-                    SpecialFormType::TypedDict.instance_fallback(db),
-                ];
 
-                safe_mutable_classes.iter().any(|safe_mutable_class| {
-                    value_ty.is_equivalent_to(db, *safe_mutable_class)
-                        || value_ty
-                            .generic_origin(db)
-                            .zip(safe_mutable_class.generic_origin(db))
-                            .is_some_and(|(l, r)| l == r)
-                })
-            };
-
-            if !value_ty.is_typed_dict() && !is_safe_mutable_class() {
+            if !value_ty.is_typed_dict() && !is_safe_mutable_class(db, value_ty) {
                 bound_ty = declared_ty;
             }
         }