A cron job stored with `provider: "custom"` and a matching `providers.custom`
entry in config failed at execution with `auth_unavailable: providers=codex`.
Two layers conspired:
- `_get_named_custom_provider` returned None for bare "custom" *before*
scanning config, so a literal `providers.custom` entry was never matched and
resolution fell through to the global default (codex). Now it scans config
for an entry literally named "custom"; with none it still returns None,
preserving the legacy model.base_url trust path.
- `_resolve_model_override` blindly stripped bare "custom" at job creation and
pinned `model.provider` (e.g. codex). It now keeps "custom" when a configured
custom endpoint resolves, pinning the main provider only when it doesn't.
xxxigm
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