Test 4a: wire NoMoreDeltas + add UpstreamSendLoop stop-and-empty exit

Code-review feedback from Task 2:
- Use state.NoMoreDeltas in the output callback completion check instead
  of downstream.Reader.Completion.IsCompleted (spec called for it as the
  redundant fast path; it was dead code before).
- UpstreamSendLoop now break's when StopSending && upstream channel empty,
  matching the spec's stated exit shape.

tests/04a-realtime-oneshot-cs/Program.cs

@@ -323,10 +323,12 @@ static Stream OpenOutputStream(int device, Channel<short[]> downstream, SharedSt
             for (int i = fill; i < (int)frameCount; i++) dst[i] = 0;
         }
 
-        // Completion = working chunk drained AND channel empty AND writer completed.
+        // Completion = working chunk drained AND channel empty AND no more deltas expected.
+        // _noMoreDeltas is the spec's "redundant fast path" — set immediately on response.done,
+        // avoids touching downstream.Reader.Completion (a Task allocation per callback).
         if (state.CurrentChunk == null
             && downstream.Reader.Count == 0
-            && downstream.Reader.Completion.IsCompleted)
+            && state.NoMoreDeltas)
         {
             doneFlag.Set();
             return StreamCallbackResult.Complete;
@@ -343,7 +345,13 @@ static async Task UpstreamSendLoop(ClientWebSocket ws, Channel<short[]> upstream
     {
         await foreach (var frame in upstream.Reader.ReadAllAsync(cts.Token))
         {
-            if (state.StopSending) continue;
+            if (state.StopSending)
+            {
+                // After server VAD said the user is done, drop in-flight mic frames and exit
+                // once the channel is drained. Matches the spec's "_stopSending && empty" exit.
+                if (upstream.Reader.Count == 0) break;
+                continue;
+            }
             ReadOnlySpan<byte> bytes = MemoryMarshal.AsBytes(frame.AsSpan());
             string base64 = Convert.ToBase64String(bytes);
             var msg = new { type = "input_audio_buffer.append", audio = base64 };