test(selftest): pin the sidebar active-surface invariant

Drives the real Sidebar.Draw and reads the render counter, so a regression in
the draw path fails instead of a parallel calculation passing.

Three cases: one of two tabs active draws exactly one surface, a null active
tab draws zero, and icon-only mode still marks the active row.

Zero is a legitimate state, not a failure -- PickMainActiveTab returns null
when every tab is popped out, so the invariant is "at most one", not "exactly
one".

HellionChat/Plugin.cs

@@ -454,6 +454,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.TellResetOnActivateStep(),
                 new SelfTests.CurrentTabCouplingStep(this),
                 new SelfTests.SidebarUnreadDotStep(this),
+                new SelfTests.SidebarActiveSurfaceStep(this),
                 new SelfTests.UnreadDecisionStep(),
                 new SelfTests.CurrentTabGuidedStep(this),
                 new SelfTests.CardClipPlanStep(this),