[codex] Propagate rate limit reached type (#18227)

## Summary

First PR in the split from #17956.

- adds the core/app-server `RateLimitReachedType` shape
- maps backend `rate_limit_reached_type` into Codex rate-limit snapshots
- carries the field through app-server notifications/responses and
generated schemas
- updates existing constructors/tests for the new optional field

## Validation

- `cargo test -p codex-backend-client`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server rate_limits`
- `cargo test -p codex-tui workspace_`
- `cargo test -p codex-tui status_`
- `just fmt`
- `just fix -p codex-backend-client`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fix -p codex-tui`

codex-rs/app-server/src/bespoke_event_handling.rs

@@ -4138,6 +4138,7 @@ mod tests {
                 balance: Some("5".to_string()),
             }),
             plan_type: None,
+            rate_limit_reached_type: None,
         };
 
         handle_token_count_event(

codex-rs/app-server/src/outgoing_message.rs

@@ -736,6 +736,7 @@ mod tests {
                     secondary: None,
                     credits: None,
                     plan_type: Some(PlanType::Plus),
+                    rate_limit_reached_type: None,
                 },
             });
 
@@ -754,7 +755,8 @@ mod tests {
                         },
                         "secondary": null,
                         "credits": null,
-                        "planType": "plus"
+                        "planType": "plus",
+                        "rateLimitReachedType": null
                     }
                 },
             }),