fix: tighten resource-leak and null-deref hot spots

Three pre-existing upstream defects flagged by CodeRabbit, fixed in the
v1.0.0 standalone cut where we own the codebase:

- Ipc/ExtraChat.cs Dispose now unsubscribes all three IPC subscriptions
  (OverrideChannelGate, ChannelCommandColoursGate, ChannelNamesGate)
  instead of only the first; previously the latter two leaked their
  subscriptions on every plugin reload
- GameFunctions/Types/TellTarget.cs FromTarget guards against a zero
  IPlayerCharacter.Address before dereferencing the unsafe Character*
  cast; previously a missing/destroyed target object would crash the
  game on /tell construction
- GameFunctions/GameFunctions.cs ResolveTextCommandPlaceholderDetour
  null-checks the Hook reference before calling .Original instead of
  using the null-forgiving operator; defensive guard for teardown races

HellionChat/GameFunctions/GameFunctions.cs

@@ -249,9 +249,15 @@ internal unsafe class GameFunctions : IDisposable
 
     private nint ResolveTextCommandPlaceholderDetour(nint a1, byte* placeholderText, byte a3, byte a4)
     {
+        // The detour is only invoked through the hook, so the hook should
+        // never be null here, but the nullable field declaration forces us
+        // to handle the theoretical race during teardown.
+        if (ResolveTextCommandPlaceholderHook is null)
+            return nint.Zero;
+
         var placeholder = MemoryHelper.ReadStringNullTerminated((nint) placeholderText);
         if (ReplacementName == null || placeholder != Placeholder)
-            return ResolveTextCommandPlaceholderHook!.Original(a1, placeholderText, a3, a4);
+            return ResolveTextCommandPlaceholderHook.Original(a1, placeholderText, a3, a4);
 
         MemoryHelper.WriteString(PlaceholderNamePtr, ReplacementName);
         ReplacementName = null;

HellionChat/GameFunctions/Types/TellTarget.cs

@@ -30,6 +30,9 @@ public class TellTarget
 
     public unsafe void FromTarget(IPlayerCharacter target)
     {
+        if (target.Address == nint.Zero)
+            return;
+
         Name = target.Name.TextValue;
         World = target.HomeWorld.RowId;
         ContentId = ((Character*)target.Address)->ContentId;

HellionChat/Ipc/ExtraChat.cs

@@ -49,6 +49,8 @@ public sealed class ExtraChat : IDisposable
     public void Dispose()
     {
         OverrideChannelGate.Unsubscribe(OnOverrideChannel);
+        ChannelCommandColoursGate.Unsubscribe(OnChannelCommandColours);
+        ChannelNamesGate.Unsubscribe(OnChannelNames);
     }
 
     private void OnOverrideChannel(OverrideInfo info)