Build the spawn env that points the agent CLI at the mock model: the caller's
environment plus the runtime's base-URL var (→ the mock's URL) and a dummy
API key. Pure (no spawn), so it's unit-tested directly — the testable seam of
the otherwise un-coverable real-subprocess path.
Build the spawn env that points the agent CLI at the mock model: the caller's environment plus the runtime's base-URL var (→ the mock's URL) and a dummy API key. Pure (no spawn), so it's unit-tested directly — the testable seam of the otherwise un-coverable real-subprocess path.