fix(asr): reject an unsupported onnx-genai language instead of decoding it as English

[jedzill4]

Stacked on #95 (base feat/canonical-language-hint, which itself sits on #92); retarget to main as the stack merges.

Problem

onnx-genai's _lang_id_for returned the English lang_id for any language the model does not list, and matched case-sensitively. So language=xx, and even ES-es, produced an English decode of the audio with HTTP 200: a wrong transcript with no error.

Change (behaviour change, agreed with the maintainer)

_lang_id_for matches the canonical spelling (#95): exact locale first, then base language (es-AR -> es), and otherwise raises AsrUnsupportedLanguageError(canonical, supported_languages=<model's languages>), which the OpenAI, SSE and Deepgram surfaces already report as a 400 with param=language. The check runs before the request takes an admission slot or touches the model. No language (None/blank) still keeps the English default.

What now fails that used to "work": a client sending a language outside the model's 40 locales used to get an English transcript; it now gets a 400 listing the supported set.

Tests (test-first)

tests/backends/asr/test_onnx_genai_backend.py:

• The test that pinned _lang_id_for("xx") == 0 now requires AsrUnsupportedLanguageError carrying the language and supported set.
• ES-es -> 2 and en_gb -> 1 (both silently decoded as English before).
• transcribe_pcm(language="xx-YY") raises before reaching the model (object() stands in for it).
• es-AR -> es, None / "" -> English default unchanged.

Local: 532 passed, 9 skipped (tests/backends, tests/api); the full prek suite passed on commit, pyrefly included.

Not verified

• Not run on a real GenAI model: _lang_id_for is pure string/dict logic and the adapter test stops before the model.
• Merge note: fix(asr): let onnx-genai device=auto use CUDA when onnxruntime offers it #93 also edits the imports at the top of this test file, so expect a trivial conflict when the stack is rebased.

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