_launch_external_cron_worker hand-rolled the same hydrate -> set_secret_scope ->
set_multiplex_context(routed) sequence, and the same context-then-scope reset, that
_install_fire_secret_scope/_reset_fire_secret_scope already encode for the in-process
fire. Two copies of the ordering is how the two paths drift apart; the helper is the
one place that owns it. The only observable difference (re-setting an already-active
multiplex context for the span) is a no-op the token reset undoes.
Rename _scope_token in _run_one_job_body to _fire_scope_tokens: it has held the helper's
(scope, context) tuple since the routed-fire change, not a single token.
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