[ty] Respect MRO_NO_OBJECT_FALLBACK policy when looking up symbols on type instances (#18312)

## Summary

This should address a problem that came up while working on
https://github.com/astral-sh/ruff/pull/18280. When looking up an
attribute (typically a dunder method) with the `MRO_NO_OBJECT_FALLBACK`
policy, the attribute is first looked up on the meta type. If the meta
type happens to be `type`, we go through the following branch in
`find_name_in_mro_with_policy`:


97ff015c88/crates/ty_python_semantic/src/types.rs (L2565-L2573)

The problem is that we now look up the attribute on `object` *directly*
(instead of just having `object` in the MRO). In this case,
`MRO_NO_OBJECT_FALLBACK` has no effect in `class_member_from_mro`:


c3feb8ce27/crates/ty_python_semantic/src/types/class.rs (L1081-L1082)

So instead, we need to explicitly respect the `MRO_NO_OBJECT_FALLBACK`
policy here by returning `Symbol::Unbound`.

## Test Plan

Added new Markdown tests that explain the ecosystem changes that we
observe.

crates/ty_python_semantic/src/types.rs

@@ -2567,9 +2567,13 @@ impl<'db> Type<'db> {
             // `Type::NominalInstance(type)` is equivalent to looking up the name in the
             // MRO of the class `object`.
             Type::NominalInstance(instance) if instance.class.is_known(db, KnownClass::Type) => {
-                KnownClass::Object
-                    .to_class_literal(db)
-                    .find_name_in_mro_with_policy(db, name, policy)
+                if policy.mro_no_object_fallback() {
+                    Some(Symbol::Unbound.into())
+                } else {
+                    KnownClass::Object
+                        .to_class_literal(db)
+                        .find_name_in_mro_with_policy(db, name, policy)
+                }
             }
 
             Type::FunctionLiteral(_)