R3-335: the provider emits reasoning blocks. Read it to decide whether to render
a thinking surface at all — an empty affordance on a provider that never thinks
is worse than none. Normalized to false by the channel when a host predating
R3-335 omits it, so this is never undefined in practice.
The resolved provider's advertised abilities (SERVICE_PROVIDERS_SPEC §2.5) — read to branch/degrade (offer image upload only when
vision).