add tests to check assigning behaviors/attrs after array creation fails

tests/test_core.py

@@ -886,3 +886,13 @@ def test_shape_only_ops(fn: Callable, tmp_path_factory: pytest.TempPathFactory)
     result = fn(lazy.b)  # type: ignore
     with dask.config.set({"awkward.optimization.enabled": True}):
         result.compute()
+
+
+def test_assign_behavior(daa_p1: dak.Array) -> None:
+    with pytest.raises(TypeError, match="'mappingproxy' object does not support item assignment"):
+        daa_p1.behavior["should_fail"] = None
+
+
+def test_assign_attrs(daa_p1: dak.Array) -> None:
+    with pytest.raises(TypeError, match="'mappingproxy' object does not support item assignment"):
+        daa_p1.attrs["should_fail"] = None