fix(selftests): take TabsListLock around Config.Tabs mutations

The three sidebar/coupling steps add, insert and remove tabs straight from
the framework thread while the message worker mutates the same list under
TabsListLock. CurrentTabCouplingStep's Insert(0, ...) is the worst of them:
it shifts every index, so DropOldestTempTab can remove the wrong tab between
its index lookup and RemoveAt.

Locks sit around the individual mutations, never around a Draw call, so no
step holds the lock across rendering.

HellionChat/SelfTests/SidebarGreetedGlyphStep.cs

@@ -49,7 +49,10 @@ internal sealed class SidebarGreetedGlyphStep : ISelfTestStep
                 [ChatType.TellOutgoing] = (ChatSourceExt.All, ChatSourceExt.All),
             },
         };
-        Plugin.Config.Tabs.Add(injected);
+        // Config.Tabs is mutated by the message worker under TabsListLock; a
+        // framework-thread writer must take the same lock.
+        lock (plugin.TabsListLock)
+            Plugin.Config.Tabs.Add(injected);
         Tab? active = null;
         var width = (float)Plugin.Config.SidebarAutoSwitchThresholdPx + 100f; // expanded
         try
@@ -91,7 +94,8 @@ internal sealed class SidebarGreetedGlyphStep : ISelfTestStep
         }
         finally
         {
-            Plugin.Config.Tabs.Remove(injected);
+            lock (plugin.TabsListLock)
+                Plugin.Config.Tabs.Remove(injected);
             Plugin.Config.AutoTellTabsShowGreetedToggle = savedFlag;
             Plugin.Config.SidebarWidth = savedSidebarWidth;
         }