Assigning to DataTree.coords should support full paths

What is your issue?

When assigning to a full path with DataTree.coords, I expected to assign a coordinate to a child, not the root.

Here's the behavior on main:

In [36]: tree = DataTree(Dataset(coords={'x': 0}), children={'child': DataTree()})

In [37]: tree.coords['/child/y'] = 2

In [38]: tree
Out[38]:
<xarray.DataTree>
Group: /
│   Dimensions:   ()
│   Coordinates:
│       x         int64 8B 0
│       /child/y  int64 8B 2
└── Group: /child

Instead, I expected the result of assigning directly to the child node (without the duiplicate inherited coordinate, of course):

In [40]: tree['/child'].coords['y'] = 2

In [41]: tree
Out[41]:
<xarray.DataTree>
Group: /
│   Dimensions:  ()
│   Coordinates:
│       x        int64 8B 0
└── Group: /child
        Dimensions:  ()
        Coordinates:
            x        int64 8B 0
            y        int64 8B 2

Start by reproducing the DataTree.coords assignment shown in the issue, comparing tree.coords['/child/y'] with tree['/child'].coords['y']. Trace how full-path coordinate assignments are resolved. Done means a full path assigns the coordinate to the child node without creating the duplicate inherited coordinate at the root.