server: forward language only where the model's contract accepts it

[CryptVenture]

Split out of #369 as requested: this PR is only the language contract gate. The ASR detail fields, the backend startup probe and the unload locking follow as separate PRs.

The problem

A model whose spec does not declare language returned HTTP 500 for every transcription the WebUI sent. build_openai_transcription_request wrote request.options["language"] whenever the body carried the key, clients send that field on every request whether or not the user chose one, and spec-backed models validate their request options strictly — so the model rejected its own client:

POST /v1/audio/transcriptions
{"model":"parakeet-tdt","audio":"...wav","language":"","text":"","options":{}}
-> {"error":{"message":"unknown Parakeet TDT request option: language"}}

Parakeet-TDT could not transcribe at all from the WebUI. AudioSR, ControlFoley, MiDashengLM-Gen, PersonaPlex and HeartMuLa fail the same way through /v1/tasks/run, which folds a top-level language into the option map.

The change

The option is forwarded only when the caller actually chose a language and the model's contract accepts it. Acceptance is resolved once at registration, next to the existing accepts_reference_text flag and for the same reason — resolving it per request re-reads the embedded spec on the request thread, which costs about 0.9 s for large GGUFs.

A language the caller placed inside options is left alone and still produces the strict rejection: an explicit option is a deliberate choice, and the error is the right answer to it. /v1/tasks/run gets the same treatment through drop_unsupported_language_option, which removes only the option that route synthesised from the top-level field.

The language keeps reaching every model through text_input, so no feature is lost. qwen3_forced_aligner reads it from there, which is why this belongs on the server rather than in the client's form — hiding the input would have broken forced alignment.

Validation

Backend: Metal, Apple M4 Max, macOS 15. Server built from this branch, Parakeet-TDT (parakeet-tdt-0.6b-v3-q8_0.gguf) loaded, real requests against /v1/audio/transcriptions:

Request	Result
"language": "" at top level	200, transcript returned
"language": "en" at top level	200, transcript returned
"options": {"language": "en"}	500, unknown Parakeet TDT request option: language — unchanged, deliberate

Build and tests:

cmake -S . -B build/macos-metal-tests -DCMAKE_BUILD_TYPE=Release \
  -DAUDIOCPP_BUILD_NATIVE_MODEL_MANAGER=ON \
  -DAUDIOCPP_USE_SYSTEM_OPENSSL=ON -DOPENSSL_ROOT_DIR="$(brew --prefix openssl@3)"
cmake --build build/macos-metal-tests -j 12
ctest --test-dir build/macos-metal-tests        # 40/40

Scope

Server only. No engine, model or output behaviour changes; affects /v1/audio/transcriptions and /v1/tasks/run request construction. Families checked against the contract: Parakeet TDT, AudioSR, ControlFoley, MiDashengLM-Gen, PersonaPlex, HeartMuLa, Qwen3 forced aligner.