The xAI TTS REST endpoint (POST /v1/tts) accepts 'speed' (0.7-1.5)
and 'optimize_streaming_latency' (0/1/2) parameters, but the Hermes
built-in xAI provider was reading neither from config nor sending
either in the request body. Add them as tts.xai.speed and
tts.xai.optimize_streaming_latency config knobs (with global
tts.speed / tts.optimize_streaming_latency fallbacks).
- speed: float, clamped to 0.7-1.5. 1.0 (the API default) is omitted
from the request body to preserve the existing minimal-payload
contract.
- optimize_streaming_latency: int, clamped to 0-2. 0 (best quality,
the API default) is omitted from the request body.
Resolver order: tts.xai.<knob> overrides the global tts.<knob>.
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