fix(tell): strip stale tell state on tab activation

OnTabActivated clears the runtime tell state the game-side detour leaves on a tab (CurrentChannel tell target + partner label) when a DIFFERENT tab becomes the input surface, so a normal typed line can no longer route as a silent /tell to the old partner. Re-clicking the active tab and tabs carrying their own Tab.TellTarget binding (leg1) are preserved.

All four activation paths route through it: Sidebar, TopTabBar, ChannelPopoutPool.TryOpen, and the MainWindow draw-seed. EnsureCurrentChannel becomes a pure derive-helper reached only via OnTabActivated. Adds the TellResetOnActivateStep self-test (step count 29 -> 30).

HellionChat/Plugin.cs

@@ -405,6 +405,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.SidebarGreetedGlyphStep(this),
                 new SelfTests.SidebarSectionHeaderStep(this),
                 new SelfTests.ScrollSnapDecisionStep(this),
+                new SelfTests.TellResetOnActivateStep(),
             ]);
 
             // Re-surface the wizard for existing users when a major UX