feat(sound): expand Simple prompts with chat (#12772)

* feat(sound): expand simple prompts with chat models

Allow callers to select an installed chat model for sound prompt
expansion. Validate its JSON before audio generation and preserve
explicit instrumental and language controls. Keep backend defaults
when expansion is omitted, including native Python ACE-Step sampling.

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

* feat(sound): map expansion by loaded audio family

Map neutral expanded fields where the resolved audio family is known.
Keep unmarked requests unchanged and reject unsupported expansion before
inference. Clear YuE2 and HeartMuLa transcripts to prevent lyric fallback.

Assisted-by: Codex:gpt-6.1-sol
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* feat(sound): choose a prompt expansion model

Require an explicit Simple choice and remember it separately from chat.
Clear unavailable choices instead of silently switching language models.
Preserve the choice in history and restore it when editing a past result.
Keep Backend default and Advanced requests compatible with existing APIs.

Document family support and align the published optional request schema.
Verify selection, request fields, persistence and history in the browser.

Assisted-by: Codex:gpt-6.1-sol
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix(sound): authorize both generation models

Check the resolved audio model and explicit expansion model before
loading either. Permission lookup failures stop generation.

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

* fix(sound): enforce JSON during prompt expansion

Constrain expansion output with the existing JSON grammar before decoding.
Require a llama-cpp chat model so unsupported backends cannot ignore it.
Keep strict field validation and fail before generating audio.

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

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

backend/cpp/audio-cpp/generation_request.h

@@ -70,9 +70,12 @@ build_tts_request(const backend::TTSRequest &request,
 
 // `source_audio` is SoundGenerationRequest.src already read, present exactly
 // when the field was set and non-empty. Same reasoning as above.
+// family comes from LoadedModel::family(), never from the request model name.
+// Marked expansions reject families without a verified field mapping.
 engine::runtime::TaskRequest
 build_sound_generation_request(const backend::SoundGenerationRequest &request,
-                               std::optional<engine::runtime::AudioBuffer> source_audio);
+                               std::optional<engine::runtime::AudioBuffer> source_audio,
+                               const std::string &family = "");
 
 // Lifts a text-conditioned transform route's text out of the request params
 // into TaskRequest.text_input, and reports whether it set one.