## Why
Follow-up to #16288: the new dynamic provider auth token flow currently
defaults `refresh_interval_ms` to a non-zero value and rejects `0`
entirely.
For command-backed bearer auth, `0` should mean "never auto-refresh".
That lets callers keep using the cached token until the backend actually
returns `401 Unauthorized`, at which point Codex can rerun the auth
command as part of the existing retry path.
## What changed
- changed `ModelProviderAuthInfo.refresh_interval_ms` to accept `0` and
documented that value as disabling proactive refresh
- updated the external bearer token refresher to treat
`refresh_interval_ms = 0` as an indefinitely reusable cached token,
while still rerunning the auth command during unauthorized recovery
- regenerated `core/config.schema.json` so the schema minimum is `0` and
the new behavior is described in the field docs
- added coverage for both config deserialization and the no-auto-refresh
plus `401` recovery behavior
## How tested
- `cargo test -p codex-protocol`
- `cargo test -p codex-login`
- `cargo test -p codex-core test_deserialize_provider_auth_config_`
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