(xai stt): expose vad_threshold, smart_turn, smart_turn_timeout, and keyterm params

[ajayarora1235]

Fixes #6588.

Follows the pattern of #5493 (which exposed endpointing): adds the remaining xAI STT websocket parameters documented at https://docs.x.ai/developers/model-capabilities/audio/speech-to-text as constructor args / update_options on xai.STT:

• vad_threshold (0.0–1.0) — built-in VAD sensitivity; xAI's websocket default is 0.08
• smart_turn (0.0–1.0) — confidence threshold for xAI's ML end-of-turn ("Smart Turn") prediction
• smart_turn_timeout (1–5000 ms) — silence backstop when Smart Turn predicts the speaker is continuing
• keyterm (list, max 100 terms) — vocabulary biasing; sent as repeated query params, so the websocket params dict became a list of pairs

All four are NotGivenOr and omitted from the query string when unset, so existing behavior is unchanged — the default connection sends the identical query as before. Verified with ruff check / ruff format; param names/ranges are taken from the xAI docs above.

Motivation: these are the main turn-taking tunables for using xai/stt-1 with turn_detection="stt" in an AgentSession — today the plugin (and the Inference gateway's xai.ExtraSettings, which rejects these keys) only exposes endpointing, so smart-turn thresholds and keyword boosting are unreachable without patching the plugin.

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[tinalenguyen]

thanks for the PR!