server : add realtime conversation and transcription on /v1/realtime (#31269)

* server : add realtime conversation and transcription on /v1/realtime

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>
Assisted-by: Claude Opus 5.5

* examples : default the realtime client to the server port 9931

Assisted-by: Claude Opus 5.5

* server : sample realtime responses right after their own decode

Slots respond in turns on one context: sampling at the start of the next step read the logits of whichever slot decoded last, so concurrent sessions mixed their outputs.

Assisted-by: Claude Opus 5.5

* move more stuff to server-realtime, making it self-contained

* examples : fix the type check of the realtime client

Assisted-by: Claude Opus 5.5

* examples : use the model card prompts in the realtime client

Assisted-by: Claude Opus 5.5

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

tools/server/server-task.cpp

@@ -1511,6 +1511,16 @@ json server_task_result_rerank::to_json() {
 //
 // server_task_result_tts
 //
+json server_task_result_realtime::to_json() {
+    return json {
+        {"text",          text},
+        {"n_samples",     audio.size()},
+        {"response_done", response_done},
+        {"cancelled",     cancelled},
+        {"n_decoded",     n_decoded},
+    };
+}
+
 json server_task_result_tts::to_json() {
     return json {
         {"sample_rate", sample_rate},