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/protocol/src/config_types.rs

@@ -283,8 +283,9 @@ pub struct ModelProviderAuthInfo {
     pub timeout_ms: NonZeroU64,
 
     /// 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.
     #[serde(default = "default_provider_auth_refresh_interval_ms")]
-    pub refresh_interval_ms: NonZeroU64,
+    pub refresh_interval_ms: u64,
 
     /// Working directory used when running the token command.
     #[serde(default = "default_provider_auth_cwd")]
@@ -297,8 +298,8 @@ impl ModelProviderAuthInfo {
         Duration::from_millis(self.timeout_ms.get())
     }
 
-    pub fn refresh_interval(&self) -> Duration {
-        Duration::from_millis(self.refresh_interval_ms.get())
+    pub fn refresh_interval(&self) -> Option<Duration> {
+        NonZeroU64::new(self.refresh_interval_ms).map(|value| Duration::from_millis(value.get()))
     }
 }
 
@@ -309,11 +310,8 @@ fn default_provider_auth_timeout_ms() -> NonZeroU64 {
     )
 }
 
-fn default_provider_auth_refresh_interval_ms() -> NonZeroU64 {
-    non_zero_u64(
-        DEFAULT_PROVIDER_AUTH_REFRESH_INTERVAL_MS,
-        "model_providers.<id>.auth.refresh_interval_ms",
-    )
+fn default_provider_auth_refresh_interval_ms() -> u64 {
+    DEFAULT_PROVIDER_AUTH_REFRESH_INTERVAL_MS
 }
 
 fn non_zero_u64(value: u64, field_name: &str) -> NonZeroU64 {