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/CurrentTabCouplingStep.cs

@@ -69,7 +69,10 @@ internal sealed class CurrentTabCouplingStep : ISelfTestStep
             // Insert a victim at index 0: a regressed index-0 getter would return
             // THIS instead of ActiveTab, so ReferenceEquals would catch it.
             var victim = new Tab { Name = "selftest-coupling-victim" };
-            Plugin.Config.Tabs.Insert(0, victim);
+            // Insert shifts every index; the worker's DropOldestTempTab holds
+            // TabsListLock across its index lookup and removal.
+            lock (_plugin.TabsListLock)
+                Plugin.Config.Tabs.Insert(0, victim);
             try
             {
                 if (!ReferenceEquals(_plugin.CurrentTab, _plugin.MainWindow.ActiveTab))
@@ -97,7 +100,8 @@ internal sealed class CurrentTabCouplingStep : ISelfTestStep
             }
             finally
             {
-                Plugin.Config.Tabs.Remove(victim);
+                lock (_plugin.TabsListLock)
+                    Plugin.Config.Tabs.Remove(victim);
             }
         }
         finally

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;
         }

HellionChat/SelfTests/SidebarSectionHeaderStep.cs

@@ -50,8 +50,13 @@ internal sealed class SidebarSectionHeaderStep : ISelfTestStep
         }
         injected.Add(BuildTempProbe("Tell Probe@SelfTest", pinned: false));
         injected.Add(BuildTempProbe("Pinned Probe@SelfTest", pinned: true));
-        foreach (var tab in injected)
-            Plugin.Config.Tabs.Add(tab);
+        // Config.Tabs is mutated by the message worker under TabsListLock; a
+        // framework-thread writer must take the same lock.
+        lock (plugin.TabsListLock)
+        {
+            foreach (var tab in injected)
+                Plugin.Config.Tabs.Add(tab);
+        }
 
         Tab? active = null;
         var width = (float)Plugin.Config.SidebarAutoSwitchThresholdPx + 100f; // expanded
@@ -84,8 +89,11 @@ internal sealed class SidebarSectionHeaderStep : ISelfTestStep
         }
         finally
         {
-            foreach (var tab in injected)
-                Plugin.Config.Tabs.Remove(tab);
+            lock (plugin.TabsListLock)
+            {
+                foreach (var tab in injected)
+                    Plugin.Config.Tabs.Remove(tab);
+            }
             Plugin.Config.AutoTellTabsCompactDisplay = savedCompact;
             Plugin.Config.SidebarWidth = savedSidebarWidth;
         }