Move the timeout coercion into src.config as coerce_provider_timeout and
run it from a field validator on ModelOverrideSettings.provider_params, so
a bad value in config.toml/env fails at startup with the exact config path
instead of surfacing per-request as a retried 500. Good values normalize
to float seconds at load. The per-request guard in src.llm.backend now
delegates to the same coercion (wrapping ValueError in ValidationException)
and continues to cover extra_params passed programmatically.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>