Test 4a: migrate session.update + event names to GA Realtime API

Hardware verification surfaced four shape changes from the beta API that
the spec/plan was written against:

1. Remove the OpenAI-Beta: realtime=v1 header (GA rejects it with
   beta_api_shape_disabled).
2. session.update payload nests under session.audio.{input,output} instead
   of flat fields. session.type = "realtime" + session.model are inline.
3. session.audio.{input,output}.format is now an object
   ({ type: "audio/pcm", rate: 24000 }), not a bare "pcm16" string.
4. 'session.modalities' is no longer accepted; dropped (server uses defaults).
5. response.audio.*  →  response.output_audio.*  (delta, done, transcript).

Also silenced a bunch of GA framing events (conversation.item.added /
.done, response.output_item.added / .done, response.content_part.added /
.done, rate_limits.updated) that were burying signal in the [ws] ignored
log lines.

Plus added an outer try/catch around main so a mid-startup WS error event
(which fires cts.Cancel via the receive loop) routes through cleanup
instead of crashing on the await SendSessionUpdate.

Verified on hardware: full round-trip works, t=0.44 s end-of-speech to
first audio sample arriving, assistant reply audible, clean exit.

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

@@ -36,7 +36,8 @@ var state = new SharedState();
 
 var ws = new ClientWebSocket();
 ws.Options.SetRequestHeader("Authorization", $"Bearer {apiKey}");
-ws.Options.SetRequestHeader("OpenAI-Beta", "realtime=v1");
+// No OpenAI-Beta header — the GA Realtime API rejects beta-shaped requests with
+// "beta_api_shape_disabled". Documented at developers.openai.com/api/docs/guides/realtime-websocket.
 
 Stream? inputStream = null;
 Stream? outputStream = null;
@@ -107,6 +108,17 @@ try
 
     Console.WriteLine("[ws] closing");
 }
+catch (OperationCanceledException)
+{
+    // Mid-startup cancellation (typically a WS error event fired cts.Cancel while
+    // the main thread was awaiting SendSessionUpdate / a channel write).
+    // The receive loop has already logged the underlying [error]; we just route through cleanup.
+}
+catch (Exception ex)
+{
+    Console.Error.WriteLine($"[error] unexpected: {ex.GetType().Name}: {ex.Message}");
+    Environment.ExitCode = 7;
+}
 finally
 {
     // Cancel any background work first so receive/send loops exit cleanly.
@@ -209,23 +221,40 @@ static short[] MakeBeep(double freqHz, double durationS, int sampleRate)
 
 static async Task SendSessionUpdate(ClientWebSocket ws, CancellationToken ct)
 {
+    // GA Realtime API shape: nested session.audio.{input,output} with format + voice + turn_detection
+    // moved under audio.input. session.type = "realtime" and session.model inline.
+    // (Beta-era flat shape with session.input_audio_format/turn_detection at the top is rejected.)
     var msg = new
     {
         type = "session.update",
         session = new
         {
-            modalities = new[] { "audio", "text" },
+            type = "realtime",
+            model = C.ModelName,
             instructions = C.Instructions,
-            voice = C.Voice,
+            // No 'modalities' field — the beta name. GA defaults are fine; if we ever need to
-            input_audio_format = "pcm16",
+            // constrain to audio-only we'd use 'output_modalities' (the documented GA name in
-            output_audio_format = "pcm16",
+            // newer OpenAI APIs).
-            turn_detection = new
+            audio = new
             {
-                type = "server_vad",
+                input = new
-                threshold = 0.5,
+                {
-                prefix_padding_ms = 300,
+                    // GA shape: format is now an object with a "type" field, not a bare string.
-                silence_duration_ms = 500,
+                    format = new { type = "audio/pcm", rate = 24_000 },
-                create_response = true,
+                    turn_detection = new
+                    {
+                        type = "server_vad",
+                        threshold = 0.5,
+                        prefix_padding_ms = 300,
+                        silence_duration_ms = 500,
+                        create_response = true,
+                    },
+                },
+                output = new
+                {
+                    format = new { type = "audio/pcm", rate = 24_000 },
+                    voice = C.Voice,
+                },
             },
         },
     };
@@ -418,11 +447,12 @@ static async Task ReceiveLoop(ClientWebSocket ws, Channel<short[]> downstream, C
                 case "response.created":
                     Console.WriteLine("[ws] response.created");
                     break;
-                case "response.audio_transcript.delta":
+                // GA event names are response.output_audio* (beta was response.audio*).
+                case "response.output_audio_transcript.delta":
                     if (doc.RootElement.TryGetProperty("delta", out var tEl))
                         transcript.Append(tEl.GetString());
                     break;
-                case "response.audio.delta":
+                case "response.output_audio.delta":
                 {
                     string b64 = doc.RootElement.GetProperty("delta").GetString() ?? "";
                     byte[] audioBytes = Convert.FromBase64String(b64);
@@ -439,8 +469,21 @@ static async Task ReceiveLoop(ClientWebSocket ws, Channel<short[]> downstream, C
                     await downstream.Writer.WriteAsync(samples, cts.Token);
                     break;
                 }
-                case "response.audio.done":
+                case "response.output_audio.done":
-                    Console.WriteLine("[ws] response.audio.done");
+                    Console.WriteLine("[ws] response.output_audio.done");
+                    break;
+                case "response.output_audio_transcript.done":
+                    // transcript already accumulated; nothing to do here, server signals end of stream.
+                    break;
+                // GA emits a stream of conversation/response framing events we don't need for the
+                // probe — quiet them so they don't bury real signal in the log.
+                case "conversation.item.added":
+                case "conversation.item.done":
+                case "response.output_item.added":
+                case "response.output_item.done":
+                case "response.content_part.added":
+                case "response.content_part.done":
+                case "rate_limits.updated":
                     break;
                 case "response.done":
                     state.NoMoreDeltas = true;