fix(audience-tests): replace Assert.DoesNotThrowAsync with await for Unity NUnit compat

Unity's com.unity.test-framework ships NUnit 3.5, which predates
Assert.DoesNotThrowAsync. Awaiting the task achieves the same assertion
semantic — NUnit fails the test if the awaited task throws.

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

Author: ImmutableJeffrey

src/Packages/Audience/Tests/Runtime/Transport/HttpTransportTests.cs

@@ -460,7 +460,7 @@ public async Task SendBatchAsync_ErrorCallbackThrows_DoesNotCrash()
                 onError: _ => throw new InvalidOperationException("callback bug"),
                 handler: handler);
 
-            Assert.DoesNotThrowAsync(() => transport.SendBatchAsync());
+            await transport.SendBatchAsync();
         }
 
 #if IMMUTABLE_AUDIENCE_GZIP