[red-knot] Infer lambda return type as Unknown (#16695)

## Summary

Part of #15382

This PR infers the return type `lambda` expression as `Unknown`. In the
future, it would be more useful to infer the expression type considering
the surrounding context (#16696).

## Test Plan

Update existing test cases from `@todo` to the (verified) return type.

crates/red_knot_python_semantic/src/types/infer.rs

@@ -3840,9 +3840,12 @@ impl<'db> TypeInferenceBuilder<'db> {
             Parameters::empty()
         };
 
+        // TODO: Useful inference of a lambda's return type will require a different approach,
+        // which does the inference of the body expression based on arguments at each call site,
+        // rather than eagerly computing a return type without knowing the argument types.
         Type::Callable(CallableType::General(GeneralCallableType::new(
             self.db(),
-            Signature::new(parameters, Some(todo_type!("lambda return type"))),
+            Signature::new(parameters, Some(Type::unknown())),
         )))
     }