fix(central-ui): InstancePicker — keyboard activation selects/toggles through TreeView selection; aria-label/InputId; aria-controls only while open

ArrowDown moved focus into the tree, but the picker passed no Selectable /
SelectedKeyChanged, so Enter/Space on a treeitem only moved the roving tabindex —
the pick lived on the label span's @onclick, i.e. mouse only. The tree now runs in
Selectable mode and BOTH gestures arrive at one handler (OnTreeKeyActivatedAsync)
through TreeView's SelectedKeyChanged: an `i:{id}` key selects that instance, an
`a:{id}` key toggles the branch. The label's own @onclick is gone — the click
already bubbles into TreeView's content span, so a second handler there would fire
the callback twice. SelectedKey stays unbound (a combobox shows its pick in the
field, not as a row highlight, and binding it would arm TreeView's
selected-key-not-in-tree reset on every render while the pick is filtered out);
SelectedCssClass is emptied to match.

a11y: the field gains an accessible name via a new AriaLabel parameter (default
"Instance") and an optional InputId so a page can point a <label for> at it;
aria-controls is rendered only while the panel exists, instead of always pointing
at an id that is absent when closed.

Also drops the unused @using of Commons.Entities.Instances from the .razor.

Tests: keyboard activation on an instance row and on an area row, a
one-selection-per-gesture guard across mouse and keyboard, and the labelling
assertions. CentralUI suite 1331 passed / 0 failed; build 0 warnings.

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Shared/InstancePickerTests.cs

@@ -65,6 +65,10 @@ public class InstancePickerTests : BunitContext
     private static string[] Rows(IRenderedComponent<InstancePicker> cut) =>
         cut.FindAll("[data-test='t-row']").Select(r => r.TextContent.Trim()).ToArray();
 
+    /// <summary>The nth treeitem in document order - i.e. the nth row the operator can see.</summary>
+    private static AngleSharp.Dom.IElement TreeItem(IRenderedComponent<InstancePicker> cut, int index) =>
+        cut.FindAll("[role='treeitem']")[index];
+
     [Fact]
     public void Closed_ShowsNothingButTheField()
     {
@@ -450,4 +454,106 @@ public class InstancePickerTests : BunitContext
         Assert.Single(cut.FindAll("[data-test='t-panel']"));
         Assert.Equal(new[] { "Zulu" }, Rows(cut));
     }
+
+    // -- Keyboard activation on the tree itself --------------------------------
+
+    [Fact]
+    public void KeyboardActivation_OnATreeRow_SelectsTheInstance()
+    {
+        int? picked = null;
+        var cut = RenderPicker(onChanged: id => picked = id);
+
+        Open(cut);
+        Press(cut, "ArrowDown");
+
+        // Areas render before root instances, so Zulu is the SECOND treeitem - the first
+        // one is the Line1 area (see KeyboardActivation_OnAnAreaRow_TogglesIt).
+        TreeItem(cut, 1).KeyDown(new KeyboardEventArgs { Key = "Enter" });
+
+        Assert.Equal(8, picked);
+        Assert.Empty(cut.FindAll("[data-test='t-panel']"));
+        Assert.Equal("Zulu", InputValue(cut));
+    }
+
+    [Fact]
+    public void KeyboardActivation_OnAnAreaRow_TogglesIt()
+    {
+        int? picked = null;
+        var cut = RenderPicker(onChanged: id => picked = id);
+
+        Open(cut);
+        Press(cut, "ArrowDown");
+
+        TreeItem(cut, 0).KeyDown(new KeyboardEventArgs { Key = "Enter" });
+
+        // An area activates as an expand, not as a pick: its children appear and the
+        // panel stays open with no callback raised.
+        Assert.Contains("Cell A", Rows(cut));
+        Assert.Single(cut.FindAll("[data-test='t-panel']"));
+        Assert.Null(picked);
+
+        // The same key on the same row closes it again - activation is a toggle.
+        TreeItem(cut, 0).KeyDown(new KeyboardEventArgs { Key = "Enter" });
+        Assert.DoesNotContain("Cell A", Rows(cut));
+    }
+
+    [Fact]
+    public void KeyboardActivation_AndMouseClick_RaiseExactlyOneSelection()
+    {
+        // The label click and Enter both run through TreeView's single selection path,
+        // so neither input mode may double-fire the callback.
+        var picks = new List<int>();
+        var cut = RenderPicker(onChanged: picks.Add);
+
+        Open(cut);
+        ClickRow(cut, "Zulu");
+        Assert.Equal(new[] { 8 }, picks);
+
+        Open(cut);
+        TreeItem(cut, 1).KeyDown(new KeyboardEventArgs { Key = "Enter" });
+        Assert.Equal(new[] { 8, 8 }, picks);
+    }
+
+    // -- Accessible name / labelling -------------------------------------------
+
+    [Fact]
+    public void Input_CarriesAnAccessibleName_ByDefaultAndFromTheParameter()
+    {
+        var cut = RenderPicker();
+        Assert.Equal("Instance", cut.Find("[data-test='t-input']").GetAttribute("aria-label"));
+
+        cut.Render(ps => ps.Add(p => p.AriaLabel, "Target instance"));
+        Assert.Equal("Target instance", cut.Find("[data-test='t-input']").GetAttribute("aria-label"));
+    }
+
+    [Fact]
+    public void InputId_DefaultsToAGeneratedId_AndHonoursTheParameter()
+    {
+        // A page that wants its own <label for> supplies the id; two pickers left to the
+        // generated one must still not collide.
+        var first = RenderPicker();
+        var second = RenderPicker();
+
+        var generated = first.Find("[data-test='t-input']").Id;
+        Assert.False(string.IsNullOrEmpty(generated));
+        Assert.NotEqual(generated, second.Find("[data-test='t-input']").Id);
+
+        first.Render(ps => ps.Add(p => p.InputId, "flow-instance"));
+        Assert.Equal("flow-instance", first.Find("[data-test='t-input']").Id);
+    }
+
+    [Fact]
+    public void AriaControls_IsRenderedOnlyWhileThePanelIsOpen()
+    {
+        var cut = RenderPicker();
+
+        // Closed: the panel element does not exist, so a dangling aria-controls would
+        // point screen readers at nothing.
+        Assert.False(cut.Find("[data-test='t-input']").HasAttribute("aria-controls"));
+
+        Open(cut);
+        Assert.Equal(
+            cut.Find("[data-test='t-panel']").Id,
+            cut.Find("[data-test='t-input']").GetAttribute("aria-controls"));
+    }
 }