Fix member lookup for unions & intersections ignoring policy (#17066)

## Summary

A quick fix for how union/intersection member search ins performed in
Knot.

## Test Plan

* Added a dunder method call test for Union, which exhibits the error
* Also added an intersection error, but it is not triggering currently
due to `call` logic not being fully implemented for intersections.

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/red_knot_python_semantic/resources/mdtest/call/dunder.md

@@ -204,6 +204,28 @@ def _(flag: bool):
     reveal_type(d[0])  # revealed: str | bytes
 ```
 
+## Calling a union of types without dunder methods
+
+We add instance attributes here to make sure that we don't treat the implicit dunder calls here like
+regular method calls.
+
+```py
+def external_getitem(instance, key: int) -> str:
+    return str(key)
+
+class NotSubscriptable1:
+    def __init__(self, value: int):
+        self.__getitem__ = external_getitem
+
+class NotSubscriptable2:
+    def __init__(self, value: int):
+        self.__getitem__ = external_getitem
+
+def _(union: NotSubscriptable1 | NotSubscriptable2):
+    # error: [non-subscriptable]
+    union[0]
+```
+
 ## Calling a possibly-unbound dunder method
 
 ```py

crates/red_knot_python_semantic/src/types.rs

@@ -1963,11 +1963,17 @@ impl<'db> Type<'db> {
 
         match self {
             Type::Union(union) => union
-                .map_with_boundness(db, |elem| elem.member(db, &name).symbol)
+                .map_with_boundness(db, |elem| {
+                    elem.member_lookup_with_policy(db, name_str.into(), policy)
+                        .symbol
+                })
                 .into(),
 
             Type::Intersection(intersection) => intersection
-                .map_with_boundness(db, |elem| elem.member(db, &name).symbol)
+                .map_with_boundness(db, |elem| {
+                    elem.member_lookup_with_policy(db, name_str.into(), policy)
+                        .symbol
+                })
                 .into(),
 
             Type::Dynamic(..) | Type::Never => Symbol::bound(self).into(),