streamFunction property
The parent's provider adapter, inherited by children without a
resolvable model role. Resolved at SPAWN time, not at construction:
a host that re-authenticates or switches provider/model mid-session
(/provider, an SSO refresh) re-points its live adapter, and children
spawned afterwards must inherit the live credential — a wiring frozen
at boot would send the stale (or empty) key and 401.
Implementation
final StreamFunction Function() streamFunction;