Rename the completion_params key to first_token_timeout_ms so the unit is
explicit in the DSL, and lower the default from 60s to 10s with a 100ms-10min
range: users who enable this gate are latency-sensitive, and 60s rarely
matches that intent. The ms->s conversion happens exactly once, at the pop
point in _normalize_completion_params; everything downstream (ModelInstance
field, ContextVar, httpx read timeout) stays in seconds.