Same strings as ModelTargetInfo.toolSupport.format.
Optional healthThe id the hop's health is recorded under: for the first hop on a key or
base URL of the route's own that differs from the provider's, the
provider id with a digest of them (see providerHealthKey). Absent
means the provider id.
Per-hop effort, cache and output budget from fallbackHopOverrides; merged over the caller's options.
Readonly chainThe hop list fixed by the route's first resolve(); later calls walk it.
A hop that is ready to serve: its provider is initialised and its window is known.