[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/protocol/src/protocol.rs

@@ -2200,6 +2200,18 @@ pub struct RateLimitSnapshot {
     pub secondary: Option<RateLimitWindow>,
     pub credits: Option<CreditsSnapshot>,
     pub plan_type: Option<crate::account::PlanType>,
+    pub rate_limit_reached_type: Option<RateLimitReachedType>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Deserialize, Serialize, JsonSchema, TS)]
+#[serde(rename_all = "snake_case")]
+#[ts(rename_all = "snake_case")]
+pub enum RateLimitReachedType {
+    RateLimitReached,
+    WorkspaceOwnerCreditsDepleted,
+    WorkspaceMemberCreditsDepleted,
+    WorkspaceOwnerUsageLimitReached,
+    WorkspaceMemberUsageLimitReached,
 }
 
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]