[ty] Do not look up __init__ on instances (#18092)

## Summary

Dunder methods are never looked up on instances. We do this implicitly
in `try_call_dunder`, but the corresponding flag was missing in the
instance-construction code where we use `member_lookup_with_policy`
directly.

fixes https://github.com/astral-sh/ty/issues/322

## Test Plan

Added regression test.

crates/ty_python_semantic/src/types.rs

@@ -4516,7 +4516,8 @@ impl<'db> Type<'db> {
                 .member_lookup_with_policy(
                     db,
                     "__init__".into(),
-                    MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
+                    MemberLookupPolicy::NO_INSTANCE_FALLBACK
+                        | MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
                 )
                 .symbol
                 .is_unbound()