test: explain msg=None on the basic-1d string bad case

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-v-b

tests/test_indexing.py

@@ -235,6 +235,8 @@ def test_get_basic_selection_0d(store: StorePath, use_out: bool, value: Any, dty
         msg="unsupported selection item for basic indexing; expected integer or slice, got <class 'float'>",
         escape=True,
     ),
+    # get_basic_selection and z[...] word their errors differently for a string
+    # selection, so this case asserts only the exception type (msg=None).
     ExpectFail(
         input="foo",
         exception=IndexError,