fix(audience): flushAsync accepts CancellationToken, swallows ObjectDisposedException

- Add optional CancellationToken. Caller can cancel the gate-wait and
  the in-flight HTTP send (default is CancellationToken.None — no
  behaviour change for existing callers).
- Catch ObjectDisposedException thrown when a concurrent Shutdown
  disposed the transport mid-flush. Previously, the exception
  propagated to the awaiter.

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

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -434,8 +434,9 @@ private static void SyncConsentToBackend(AudienceConfig config, ConsentLevel lev
         // Flush / Shutdown
         // -----------------------------------------------------------------
 
-        // Send pending events now.
-        public static async Task FlushAsync()
+        // Sends all pending events now. Respects cancellationToken for both
+        // the gate wait and the HTTP send.
+        public static async Task FlushAsync(CancellationToken cancellationToken = default)
         {
             if (!_initialized) return;
 
@@ -451,16 +452,22 @@ public static async Task FlushAsync()
             // (timer SendBatch or a racing FlushAsync) holds the gate.
             while (Interlocked.CompareExchange(ref _sendInFlight, 1, 0) != 0)
             {
+                cancellationToken.ThrowIfCancellationRequested();
                 await Task.Yield();
             }
 
             try
             {
                 while (!transport.IsInBackoffWindow &&
-                       await transport.SendBatchAsync().ConfigureAwait(false))
+                       await transport.SendBatchAsync(cancellationToken).ConfigureAwait(false))
                 {
                 }
             }
+            catch (ObjectDisposedException)
+            {
+                // Concurrent Shutdown disposed the transport. Exit silently —
+                // caller is tearing down.
+            }
             finally
             {
                 Interlocked.Exchange(ref _sendInFlight, 0);