login: treat provider auth refresh_interval_ms=0 as no auto-refresh (#16480)

## 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_`

codex-rs/core/config.schema.json

@@ -845,9 +845,9 @@
         },
         "refresh_interval_ms": {
           "default": 300000,
-          "description": "Maximum age for the cached token before rerunning the command.",
+          "description": "Maximum age for the cached token before rerunning the command. Set to `0` to disable proactive refresh and only rerun after a 401 retry path.",
           "format": "uint64",
-          "minimum": 1.0,
+          "minimum": 0.0,
           "type": "integer"
         },
         "timeout_ms": {