feat: show enterprise monthly credit limits in status (#24812)

## Summary

Enterprise users can have an effective monthly credit limit, but Codex
`/status` currently drops that metadata from the account-usage response.

This change adds the optional `spend_control.individual_limit`
projection to the existing rate-limit snapshot flow. The backend client
reads the monthly limit, app-server exposes it as `individualLimit`, and
the TUI renders a `Monthly credit limit` row through the existing
progress-bar renderer.

When the backend does not return an effective monthly limit, existing
rate-limit behavior is unchanged.

## Existing backend state

The account-usage backend already returns the effective monthly limit
and current usage together:

```json
{
  "spend_control": {
    "reached": false,
    "individual_limit": {
      "limit": "25000",
      "used": "8000",
      "remaining": "17000",
      "used_percent": 32,
      "remaining_percent": 68,
      "reset_after_seconds": 86400,
      "reset_at": 1778137680
    }
  }
}
```

Before this change, Codex projected rolling `primary` and `secondary`
windows plus `credits`. It ignored `spend_control.individual_limit`, so
app-server clients and `/status` could not render the monthly cap.

The updated flow is:

```text
account usage backend
  -> backend-client reads spend_control.individual_limit
  -> existing rate-limit snapshot carries optional individual_limit
  -> app-server exposes optional individualLimit
  -> TUI renders Monthly credit limit
```

## App-server contract

`account/rateLimits/read` and sparse `account/rateLimits/updated`
notifications now include an additive nullable
`rateLimits.individualLimit` field:

```json
{
  "individualLimit": {
    "limit": "25000",
    "used": "8000",
    "remainingPercent": 68,
    "resetsAt": 1778137680
  }
}
```

In an `account/rateLimits/read` response, `null` means no monthly limit
is available. `account/rateLimits/updated` remains a sparse rolling
notification: clients merge available values into their most recent
`account/rateLimits/read` snapshot or refetch. Nullable account metadata
in a rolling notification does not clear a previously observed value.

## Design decisions

- Extend the existing rate-limit snapshot instead of introducing a
separate request or wire-level update protocol.
- Keep the Codex projection narrow: `/status` needs the effective limit,
current usage, remaining percentage, and reset timestamp.
- Render the monthly row through the existing progress-bar renderer,
with one optional detail line for `8,000 of 25,000 credits used`.
- Keep the backend response optional so existing accounts and older
usage states preserve their current behavior.
- Preserve cached monthly metadata when sparse rolling notifications
omit it. Live account-usage reads remain authoritative and can clear a
removed limit.

## Visual evidence

```text
 Monthly credit limit:   [██████████████░░░░░░] 68% left (resets 07:08 on 7 May)
                         8,000 of 25,000 credits used
```

Snapshot:
`codex-rs/tui/src/status/snapshots/codex_tui__status__tests__status_snapshot_includes_enterprise_monthly_credit_limit.snap`

## Testing

Tests: generated app-server schema verification, protocol tests,
backend-client tests, app-server integration coverage, TUI snapshot
coverage, formatting, and workspace lint cleanup.

codex-rs/app-server-protocol/src/protocol/v2/account.rs

@@ -6,6 +6,7 @@ use codex_protocol::protocol::CreditsSnapshot as CoreCreditsSnapshot;
 use codex_protocol::protocol::RateLimitReachedType as CoreRateLimitReachedType;
 use codex_protocol::protocol::RateLimitSnapshot as CoreRateLimitSnapshot;
 use codex_protocol::protocol::RateLimitWindow as CoreRateLimitWindow;
+use codex_protocol::protocol::SpendControlLimitSnapshot as CoreSpendControlLimitSnapshot;
 use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
@@ -247,6 +248,11 @@ pub struct AccountUpdatedNotification {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
+/// Sparse rolling rate-limit update.
+///
+/// Clients should merge available values into the most recent `account/rateLimits/read` response
+/// or refetch that snapshot. Nullable account metadata may be unavailable in a rolling update and
+/// does not clear a previously observed value.
 pub struct AccountRateLimitsUpdatedNotification {
     pub rate_limits: RateLimitSnapshot,
 }
@@ -260,6 +266,7 @@ pub struct RateLimitSnapshot {
     pub primary: Option<RateLimitWindow>,
     pub secondary: Option<RateLimitWindow>,
     pub credits: Option<CreditsSnapshot>,
+    pub individual_limit: Option<SpendControlLimitSnapshot>,
     pub plan_type: Option<PlanType>,
     pub rate_limit_reached_type: Option<RateLimitReachedType>,
 }
@@ -272,6 +279,7 @@ impl From<CoreRateLimitSnapshot> for RateLimitSnapshot {
             primary: value.primary.map(RateLimitWindow::from),
             secondary: value.secondary.map(RateLimitWindow::from),
             credits: value.credits.map(CreditsSnapshot::from),
+            individual_limit: value.individual_limit.map(SpendControlLimitSnapshot::from),
             plan_type: value.plan_type,
             rate_limit_reached_type: value
                 .rate_limit_reached_type
@@ -371,6 +379,28 @@ impl From<CoreCreditsSnapshot> for CreditsSnapshot {
     }
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct SpendControlLimitSnapshot {
+    pub limit: String,
+    pub used: String,
+    pub remaining_percent: i32,
+    #[ts(type = "number")]
+    pub resets_at: i64,
+}
+
+impl From<CoreSpendControlLimitSnapshot> for SpendControlLimitSnapshot {
+    fn from(value: CoreSpendControlLimitSnapshot) -> Self {
+        Self {
+            limit: value.limit,
+            used: value.used,
+            remaining_percent: value.remaining_percent,
+            resets_at: value.resets_at,
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]