Cron's job runner was the last entry point still reading
fallback_providers/fallback_model as an either/or, silently dropping the
legacy fallback_model when fallback_providers was set. Every other entry
point (cli, gateway, oneshot, fallback_cmd, tui_gateway, auxiliary_client)
already merges both keys via get_fallback_chain(). This aligns cron with
them at both call sites: the auth-fallback resolution loop and the
AIAgent(fallback_model=...) argument.
Co-authored-by: xxxigm <tuancanhnguyen706@gmail.com>
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