test(dv-3): add 4-level roll-up + deep-leaf filter tests; return AsReadOnly; add caller-contract remark

Fix 1 (Important): RollUp_FourLevelDeepBadQuality_ReachesRoot — proves bad quality at a
4-segment-deep leaf propagates HasBadQuality up every ancestor to the root.

Fix 2 (Important): Filter_DeepLeafMatch_RetainsAllAncestorBranches — proves filtering on
a terminal segment of a 3-level path retains all ancestor branches.

Fix 3 (Minor): BuildAttributeTree now returns roots.AsReadOnly() so the returned
IReadOnlyList<DebugTreeNode> reference is not a mutable list.

Fix 4 (Minor): Added <remarks> XML doc to BuildAttributeTree noting the caller-contract
that at most one AttributeValueChanged per AttributeName should be passed.

All 18 DebugTreeBuilder tests pass.

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/DebugTreeBuilder.cs

@@ -21,6 +21,13 @@ public static class DebugTreeBuilder
     /// <see cref="AttributeValueChanged.AttributeName"/>). Null/empty/whitespace
     /// keeps everything; matching leaves carry along their ancestor branches.
     /// </param>
+    /// <remarks>
+    /// The caller is expected to pass at most one <see cref="AttributeValueChanged"/>
+    /// per <see cref="AttributeValueChanged.AttributeName"/>. The DebugView page
+    /// enforces this by keying a dictionary on the attribute name before calling this
+    /// method. Passing duplicate names would produce sibling leaves with identical
+    /// keys under the same parent branch.
+    /// </remarks>
     public static IReadOnlyList<DebugTreeNode> BuildAttributeTree(
         IEnumerable<AttributeValueChanged> attributes, string? filter)
     {
@@ -71,7 +78,7 @@ public static class DebugTreeBuilder
         }
 
         SortAndRollUp(roots);
-        return roots;
+        return roots.AsReadOnly();
     }
 
     /// <summary>