Add Text to Speech support via ITextToSpeechClient

Implements ITextToSpeechClient for the xAI/Grok TTS API with support
for both unary and streaming synthesis:

- GrokTextToSpeechClient: unary via POST /v1/tts and streaming via
  WebSocket wss://.../v1/tts
- GrokTextToSpeechOptions: extends TextToSpeechOptions with
  Grok-specific parameters (SampleRate, BitRate,
  OptimizeStreamingLatency, TextNormalization)
- AsITextToSpeechClient extension on GrokClient for easy setup
- Full unit test coverage for request mapping, codec/media-type
  resolution, error handling, and streaming event processing
- readme: documents unary synthesis, streaming with progressive file
  writes, and Grok-specific options including available voices and
  audio formats

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

Author: kzu

AGENTS.md

@@ -0,0 +1,5 @@
+# xAI SDK implementation notes
+
+- `GrokClient` is primarily backed by generated gRPC protocol clients, but text to speech uses xAI's documented REST/WebSocket voice endpoints because there are no generated TTS protocol types in `src\xAI.Protocol`.
+- `AsITextToSpeechClient` returns an `ITextToSpeechClient` implementation that uses `POST /v1/tts` for unary audio and `wss://.../v1/tts` for streaming audio.
+- TTS defaults follow xAI docs: voice `eve`, language `en` when omitted by `TextToSpeechOptions`, and MP3 output when no codec is specified.

readme.md

@@ -45,6 +45,12 @@ var chat = new GrokClient(Environment.GetEnvironmentVariable("XAI_API_KEY")!)
 
 var images = new GrokClient(Environment.GetEnvironmentVariable("XAI_API_KEY")!)
     .AsIImageGenerator("grok-imagine-image");
+
+var speech = new GrokClient(Environment.GetEnvironmentVariable("XAI_API_KEY")!)
+    .AsITextToSpeechClient();
+
+var audio = await speech.GetAudioAsync("Hello! Welcome to xAI text to speech.",
+    new TextToSpeechOptions { VoiceId = "eve", Language = "en" });
 ```
 
 ## File Attachments
@@ -393,6 +399,72 @@ var editedImage = (UriContent)result.Contents.First();
 Console.WriteLine($"Edited image URL: {editedImage.Uri}");
 ```
 
+## Text to Speech
+
+Grok supports text to speech via the `ITextToSpeechClient` abstraction from Microsoft.Extensions.AI.
+Use `AsITextToSpeechClient` to get a TTS client:
+
+```csharp
+var speech = new GrokClient(Environment.GetEnvironmentVariable("XAI_API_KEY")!)
+    .AsITextToSpeechClient();
+```
+
+### Unary (single response)
+
+Call `GetAudioAsync` to synthesize speech in a single request. The result contains a `DataContent` 
+with the audio bytes and media type:
+
+```csharp
+var response = await speech.GetAudioAsync("Hello! Welcome to xAI text to speech.",
+    new TextToSpeechOptions { VoiceId = "eve", Language = "en" });
+
+var audio = (DataContent)response.Contents.First();
+// audio.MediaType == "audio/mpeg" (MP3 by default)
+await File.WriteAllBytesAsync("output.mp3", audio.Data.ToArray());
+```
+
+Available voices include `ara`, `eve`, `leo`, `rex`, and `sal`. Defaults to `eve` and English when 
+`VoiceId`/`Language` are not specified.
+
+### Streaming
+
+Call `GetStreamingAudioAsync` to receive audio chunks as they are generated, enabling low-latency 
+playback or progressive file writes:
+
+```csharp
+await using var fileStream = File.Create("output.mp3");
+
+await foreach (var update in speech.GetStreamingAudioAsync("Hello from streaming TTS!",
+    new TextToSpeechOptions { VoiceId = "eve", AudioFormat = "mp3" }))
+{
+    if (update.Kind == TextToSpeechResponseUpdateKind.AudioUpdating)
+    {
+        foreach (var content in update.Contents.OfType<DataContent>())
+            await fileStream.WriteAsync(content.Data);
+    }
+}
+```
+
+### Grok-Specific Options
+
+Use `GrokTextToSpeechOptions` to control audio quality and streaming behavior beyond the base 
+`TextToSpeechOptions`:
+
+```csharp
+var options = new GrokTextToSpeechOptions
+{
+    VoiceId = "rex",
+    Language = "en",
+    AudioFormat = "mp3",      // mp3 | wav | pcm | mulaw | alaw
+    SampleRate = 24000,       // Hz
+    BitRate = 128000,         // bits per second (MP3 only)
+    OptimizeStreamingLatency = 1,   // 0–4; higher trades quality for lower latency
+    TextNormalization = true, // expand abbreviations and numbers before synthesis
+};
+
+var response = await speech.GetAudioAsync("Streaming at 24 kHz, 128 kbps.", options);
+```
+
 <!-- #xai -->
 
 # xAI.Protocol

src/xAI.Tests/ChatClientTests.cs

@@ -21,7 +21,9 @@ public async Task OpenAIInvokesTools()
             { "user", "What day is today?" },
         };
 
-        var chat = new OpenAIClient(Configuration["OPENAI_API_KEY"]!).GetChatClient("gpt-5.4").AsIChatClient()
+        var chat = new OpenAIClient(Configuration["OPENAI_API_KEY"]!)
+            .GetChatClient("gpt-5.4")
+            .AsIChatClient()
             .AsBuilder()
             .UseFunctionInvocation(configure: client => client.MaximumIterationsPerRequest = 3)
             .UseLogging(output.AsLoggerFactory())
@@ -96,10 +98,10 @@ public async Task GrokInvokesTools()
     [SecretsFact("XAI_API_KEY")]
     public async Task GrokReasoningModelOutputsBothContentAndEncryptedReasoning()
     {
-        var grok = new GrokClient(Configuration["XAI_API_KEY"]!).AsIChatClient("grok-4-1-fast");
+        var grok = new GrokClient(Configuration["XAI_API_KEY"]!).AsIChatClient("grok-4-1-fast-reasoning");
 
         var response = await grok.GetResponseAsync(
-            "What is 3 + 4? Respond with just the number.",
+            "What is 3 + 4? Respond with just the number, think about it really well.",
             new GrokChatOptions
             {
                 UseEncryptedContent = true