fix(voice): use ResponseHeadersRead for SSE streaming

HttpClient defaults to buffering the entire response before returning
the stream. Using HttpCompletionOption.ResponseHeadersRead allows the
client to read SSE events as they arrive from the server, enabling
real-time streaming display in the app.

Also added server-side diagnostic logging for streaming events.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja

lib/McpServer

@@ -107,10 +107,16 @@ public async IAsyncEnumerable<McpVoiceTurnStreamEvent> SubmitTurnStreamingAsync(
         HttpResponseMessage? response = null;
         try
         {
-            response = await client.PostAsJsonAsync(
-                $"mcp/voice/session/{Uri.EscapeDataString(sessionId)}/turn/stream",
-                request,
-                JsonOptions,
+            var jsonBody = JsonSerializer.Serialize(request, JsonOptions);
+            using var content = new StringContent(jsonBody, System.Text.Encoding.UTF8, "application/json");
+
+            // ResponseHeadersRead is critical: starts returning stream data immediately
+            // instead of buffering the entire response
+            response = await client.SendAsync(
+                new HttpRequestMessage(HttpMethod.Post,
+                    $"mcp/voice/session/{Uri.EscapeDataString(sessionId)}/turn/stream")
+                { Content = content },
+                HttpCompletionOption.ResponseHeadersRead,
                 cancellationToken).ConfigureAwait(false);
             await EnsureSuccessAsync(response, cancellationToken).ConfigureAwait(false);