`_resolve_model_override` treated any non-empty `provider` string from
the LLM as user-specified and skipped the pin-to-current-provider
fallback. When the LLM wrote bare `'custom'` (instead of the canonical
`'custom:<name>'` referring to a custom_providers entry), the value
serialized into jobs.json as `"provider": "custom"` and the scheduler
could never resolve a provider from it — the cron job failed silently
at run time.
Treat bare `'custom'` as "no provider supplied" so the current main
provider gets pinned instead, matching behaviour for the omitted case.
Defence-in-depth complement to a schema-description fix (#15477) that
discourages the LLM from emitting bare `'custom'` in the first place.
cba86b7303