fix: dak.copy should copy metadata (#413)

* fix: make deep (meta) copy in dak.copy

* fix: only copy metadata

src/dask_awkward/lib/structure.py

@@ -3,6 +3,7 @@
 import builtins
 import warnings
 from collections.abc import Iterable, Mapping, Sequence
+from copy import deepcopy
 from numbers import Number
 from typing import TYPE_CHECKING, Any
 
@@ -315,7 +316,17 @@ def combinations(
 
 @borrow_docstring(ak.copy)
 def copy(array: Array) -> Array:
-    return array
+    # Make a copy of meta, but don't try and copy the layout;
+    # dask-awkward's copy is metadata-only
+    old_meta = array._meta
+    new_meta = ak.Array(old_meta.layout, behavior=deepcopy(old_meta._behavior))
+
+    return Array(
+        array._dask,
+        array._name,
+        new_meta,
+        array._divisions,
+    )
 
 
 class _FillNoneFn:

tests/test_structure.py

@@ -388,11 +388,8 @@ def test_sort(daa, caa, ascending):
 
 
 def test_copy(daa):
-    with pytest.raises(
-        DaskAwkwardNotImplemented,
-        match="This function is not necessary in the context of dask-awkward.",
-    ):
-        dak.copy(daa)
+    result = dak.copy(daa)
+    assert result._meta is not daa._meta
 
 
 @pytest.mark.parametrize(