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/protocol/src/error.rs

@@ -650,6 +650,8 @@ pub struct UsageLimitReachedError {
 
 impl std::fmt::Display for UsageLimitReachedError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // Reserve is a fallback for exhausted ordinary usage, so keep the standard
+        // promo/plan recovery copy below instead of suggesting another model.
         if let Some(limit_name) = self
             .rate_limits
             .as_ref()
@@ -657,6 +659,7 @@ impl std::fmt::Display for UsageLimitReachedError {
             .map(str::trim)
             .filter(|name| !name.is_empty())
             && !limit_name.eq_ignore_ascii_case("codex")
+            && !limit_name.eq_ignore_ascii_case("gpt-reserve")
         {
             return write!(
                 f,