Stateless cross-process bootstrap.
POSTs {bootstrap_path} against the running test binary with the
preconfigured token + username + password, parses the
BootstrapOutput envelope, captures the session Set-Cookie onto
the supplied FetchTransport (which carries it on every subsequent
call), and returns the keeper credentials.
Fires exactly once per backend lifetime — spawn_backend calls it
inside vitest's globalSetup. Per-test fixtures re-use the captured
keeper credentials; fresh per-test accounts come from
fixture.create_account() (signup+login through production RPC),
not re-bootstrap. The hybrid reset model in default_cross_process_setup
depends on this — re-bootstrap would race the bootstrap lock and
in-memory caches.