fix(audio): use captions as the default YuE2 style (#12703)

* fix(audio): use captions as the default YuE2 style

Studio sends a caption, but YuE2 requires a style request option.
Default audio-cpp style from caption while preserving explicit options
and the caller's parameter map. Document the Studio fields.

Assisted-by: Codex:gpt-6
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix(audio-cpp): map captions to style in backend

Move the YuE2 caption fallback into the C++ request builder as requested.
Preserve explicit style options, including empty values, and move the
regression cases from the Go layer into the backend suite.

Assisted-by: Codex:GPT-6

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

backend/cpp/audio-cpp/generation_request.cpp

@@ -232,9 +232,12 @@ build_sound_generation_request(const backend::SoundGenerationRequest &request,
         task.options["think"] = bool_option(request.think());
     }
     if (request.has_caption()) {
-        // Read only by irodori_tts, which advertises Tts/VoiceCloning/VoiceDesign
-        // and not AudioGeneration, so it is unreachable from this RPC: DEAD here.
         task.options["caption"] = request.caption();
+        // YuE2 calls the musical caption "style". Preserve explicit options,
+        // including an empty style, while accepting Studio's caption field.
+        if (!request.caption().empty()) {
+            task.options.emplace("style", request.caption());
+        }
     }
     if (request.has_lyrics()) {
         // Read by ace_step and heartmula.