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/core/src/session/tests.rs

@@ -3118,6 +3118,7 @@ async fn set_rate_limits_retains_previous_credits() {
             unlimited: false,
             balance: Some("10.00".to_string()),
         }),
+        individual_limit: None,
         plan_type: Some(codex_protocol::account::PlanType::Plus),
         rate_limit_reached_type: None,
     };
@@ -3137,6 +3138,7 @@ async fn set_rate_limits_retains_previous_credits() {
             resets_at: Some(1_900),
         }),
         credits: None,
+        individual_limit: None,
         plan_type: None,
         rate_limit_reached_type: None,
     };
@@ -3150,6 +3152,7 @@ async fn set_rate_limits_retains_previous_credits() {
             primary: update.primary.clone(),
             secondary: update.secondary,
             credits: initial.credits,
+            individual_limit: initial.individual_limit,
             plan_type: initial.plan_type,
             rate_limit_reached_type: None,
         })
@@ -3227,6 +3230,7 @@ async fn set_rate_limits_updates_plan_type_when_present() {
             unlimited: false,
             balance: Some("15.00".to_string()),
         }),
+        individual_limit: None,
         plan_type: Some(codex_protocol::account::PlanType::Plus),
         rate_limit_reached_type: None,
     };
@@ -3242,6 +3246,7 @@ async fn set_rate_limits_updates_plan_type_when_present() {
         }),
         secondary: None,
         credits: None,
+        individual_limit: None,
         plan_type: Some(codex_protocol::account::PlanType::Pro),
         rate_limit_reached_type: None,
     };
@@ -3255,6 +3260,7 @@ async fn set_rate_limits_updates_plan_type_when_present() {
             primary: update.primary,
             secondary: update.secondary,
             credits: initial.credits,
+            individual_limit: initial.individual_limit,
             plan_type: update.plan_type,
             rate_limit_reached_type: None,
         })