Skip multiple inheritance checks for "special" fields

We're skipping them for the override checks already. Re-using the
refactored function to also skip them during multiple inheritance
checks.

Newly added test is passing because we don't check methods in multiple
inheritance checks yet. Adding the test there to prevent regressions.

Author: fangyi-zhou

pyrefly/lib/alt/class/class_field.rs

@@ -1982,6 +1982,10 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         for parent_cls in mro.ancestors_no_object().iter() {
             let class_fields = parent_cls.class_object().fields();
             for field in class_fields {
+                // Skip fields that are not relevant for override consistency checks
+                if !self.should_check_field_for_override_consistency(field) {
+                    continue;
+                }
                 let key = KeyClassField(parent_cls.class_object().index(), field.clone());
                 let field_entry = self.get_from_class(cls, &key);
                 if let Some(field_entry) = field_entry.as_ref() {

pyrefly/lib/test/class_subtyping.rs

@@ -324,3 +324,16 @@ class Both(Foo, Bar): # Expect error here
     ...
 "#,
 );
+
+testcase!(
+    test_multiple_inheritance_special_methods,
+    r#"
+class Foo:
+    def __init__(self, x: int) -> None: ...
+class Bar:
+    def __init__(self, x: str) -> None: ...
+
+class Both(Foo, Bar): # No error here, because __init__ is special
+    ...
+"#,
+);