Add Luna Reserve usage fallback to the TUI (#42372)

## Why

When ordinary included usage is exhausted, eligible users need a way to keep working with the backend-provided Luna Reserve quota and return to their previous model once ordinary usage recovers.

## What changed

- Automatically switch eligible TUI tasks to Luna Reserve after an authoritative account usage read, preserving the prior model and reasoning effort per task.
- Restrict model selection while on Reserve to its supported reasoning levels, keep queued turns on the accepted model, and restore the saved model after a fresh read confirms recovery.
- Surface Reserve usage in the composer and `/status`, and expose `normalModelSlug` in app-server rate-limit snapshots so clients can use the associated model's display metadata without changing the request model.
- Poll account limits more frequently near exhaustion and retain compatibility with app servers that only accept the legacy null request parameters.

## Testing

- Added coverage for Reserve entry, queued-turn handling, task reconstruction, recovery authorization, account changes, model selection, usage rendering, and banner dismissal.

GitOrigin-RevId: 46a435e9531051f8cec3f6a46c4b9f7cfa479747

codex-rs/codex-api/src/rate_limits.rs

@@ -90,6 +90,7 @@ pub fn parse_rate_limit_for_limit(
     Some(RateLimitSnapshot {
         limit_id: Some(normalized_limit_id),
         limit_name: parsed_limit_name,
+        normal_model_slug: None,
         primary,
         secondary,
         credits,
@@ -156,6 +157,7 @@ pub fn parse_rate_limit_event(payload: &str) -> Option<RateLimitSnapshot> {
     Some(RateLimitSnapshot {
         limit_id: Some(limit_id.unwrap_or_else(|| "codex".to_string())),
         limit_name: None,
+        normal_model_slug: None,
         primary,
         secondary,
         credits,