DataTree: selecting via `[["a"]]` suggests `subset`, which is missing

What happened?

calling dt[["a"]] suggests to use .subset which does not exist.

What did you expect to happen?

No response

Minimal Complete Verifiable Example

import xarray as xr
a = xr.Dataset(data_vars={"x": [10, 20]}, coords={"time": [0, 1]})

dt = xr.DataTree()
dt["a"] = a

dt[["a"]] # fails and suggests subset
dt.subset # fails

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

Start by running the provided Python example with xarray.DataTree, focusing on dt[["a"]] and the suggested dt.subset attribute. Trace the DataTree indexing and suggestion behavior, then confirm the completed fix no longer suggests a missing subset attribute for this selection.