Release local publication and Rtc source FFI handles on cleanup

After the previous commit, Room cleanup walked participants and disposed
their handles, including the publication handles for remote tracks. Three
local-side handles still leaked on every disconnect:

- LocalTrackPublication never wrapped its FFI handle. When PublishTrack
  succeeded, OnPublish constructed the C# publication from the proto info
  alone and dropped e.Publication.Handle on the floor. The Rust side kept
  the entry alive in the FFI handle table for the rest of the process.

- RtcVideoSource and RtcAudioSource owned an FFI source handle but their
  Dispose(bool) implementations released the preview texture, capture
  buffer, and pending audio frames without ever disposing the handle. The
  source therefore stayed registered with Rust until the SafeHandle
  finalizer eventually ran.

Wrap the publication handle in the PublishTrackInstruction callback and
dispose it through the existing DisposeHandles cascade. Add Handle
disposal to the two Rtc source Dispose(bool) overrides so the Meet
sample's CleanUpAllTracks now actually frees them.

With this change, disconnecting after publishing local mic + camera
returns the FFI handle table to its pre-connect baseline on macOS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MaxHeimbrock

Runtime/Scripts/Participant.cs

@@ -624,7 +624,7 @@ internal void OnPublish(PublishTrackCallback e)
 
             IsError = !string.IsNullOrEmpty(e.Error);
             IsDone = true;
-            var publication = new LocalTrackPublication(e.Publication.Info);
+            var publication = new LocalTrackPublication(e.Publication.Info, FfiHandle.FromOwnedHandle(e.Publication.Handle));
             publication.UpdateTrack(_localTrack as Track);
             _localTrack.UpdateSid(publication.Sid);
             _internalTracks.Add(e.Publication.Info.Sid, publication);

Runtime/Scripts/RtcAudioSource.cs

@@ -294,6 +294,7 @@ protected virtual void Dispose(bool disposing)
                 }
                 _pendingFrameData.Clear();
             }
+            Handle?.Dispose();
             _disposed = true;
             Utils.Debug($"{DebugTag} disposed");
         }

Runtime/Scripts/RtcVideoSource.cs

@@ -211,6 +211,7 @@ protected virtual void Dispose(bool disposing)
                 Debug.Log("Disposing capture buffer");
                 _captureBuffer.Dispose();
             }
+            Handle?.Dispose();
             _disposed = true;
         }
 

Runtime/Scripts/TrackPublication.cs

@@ -96,8 +96,17 @@ public sealed class LocalTrackPublication : TrackPublication
     {
         public new ILocalTrack Track => base.Track as ILocalTrack;
 
-        internal LocalTrackPublication(TrackPublicationInfo info) : base(info)
+        private FfiHandle Handle;
+
+        internal LocalTrackPublication(TrackPublicationInfo info, FfiHandle handle) : base(info)
         {
+            Handle = handle;
+        }
+
+        internal override void DisposeHandles()
+        {
+            base.DisposeHandles();
+            Handle?.Dispose();
         }
     }
 }