test(immutable): assert length snapshot behavior

`ImmutableList` does not store a `_length` attribute; length is derived from
the immutable `_items` tuple. The previous test passed by trying to assign a
nonexistent frozen dataclass field, which did not verify any real length state.

Replace it with a behavior-focused assertion: constructing from a mutable list
takes a tuple snapshot, so later mutations to the source list do not affect
`len()`.

No runtime behavior change; the full tox matrix (lint, mypy strict, unit tests
on Python 3.10-3.14) passes.

Author: hakandilek

tests/test_immutable_list.py

@@ -84,9 +84,12 @@ def test_immutable_items_attribute(self) -> None:
             il.__setattr__('_items', 10)
 
     def test_immutable_length(self) -> None:
-        il = ImmutableList(1, 2, 3, 4, 5)
-        with self.assertRaises(FrozenInstanceError):
-            il.__setattr__('_length', 10)
+        items = [1, 2, 3, 4, 5]
+        il = ImmutableList[int](items)
+
+        items.append(6)
+
+        self.assertEqual(len(il), 5)
 
     def test_map_values_from_a_tuple(self) -> None:
         values = (1, 2, 3, 4, 5)