feat(app-server): expose rate-limit reset credits (#28143)

## Why

Codex users can earn personal rate-limit reset credits, but app-server
clients do not currently have an API for reading or redeeming them. This
adds the backend and protocol foundation used by the `/usage` TUI flow
in #28154.

## What changed

- Extend `account/rateLimits/read` with a nullable
`rateLimitResetCredits` summary sourced from the existing usage
response.
- Add backend-client and app-server support for consuming a reset with a
caller-generated idempotency key. A UUID is recommended, and clients
reuse the same key when retrying the same logical reset.
- Return only the consume `outcome`; clients refetch
`account/rateLimits/read` for updated window state.
- Document the response field and each consume outcome, and regenerate
the JSON and TypeScript schema fixtures.
- Clarify in `AGENTS.md` that new app-server string enum values use
camelCase on the wire.
- Update the existing TUI response fixture for the expanded protocol
shape.
- Add coverage for authentication, response mapping, backend failures,
consume outcomes, and request timeout behavior.

## Validation

- `just test -p codex-app-server-protocol` — 231 passed.
- `just test -p codex-backend-client` — 14 passed.
- Focused `codex-app-server` reset-credit tests — 5 passed.
- Focused `codex-tui` protocol response fixture test — passed.
- `just fix -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server` — passed.
- `just fmt` — passed.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -1004,6 +1004,12 @@ client_request_definitions! {
         response: v2::GetAccountRateLimitsResponse,
     },
 
+    ConsumeAccountRateLimitResetCredit => "account/rateLimitResetCredit/consume" {
+        params: v2::ConsumeAccountRateLimitResetCreditParams,
+        serialization: global("account-auth"),
+        response: v2::ConsumeAccountRateLimitResetCreditResponse,
+    },
+
     GetAccountTokenUsage => "account/usage/read" {
         params: #[ts(type = "undefined")] #[serde(skip_serializing_if = "Option::is_none")] Option<()>,
         serialization: None,

codex-rs/app-server-protocol/src/protocol/v2/account.rs

@@ -256,6 +256,44 @@ pub struct GetAccountRateLimitsResponse {
     pub rate_limits: RateLimitSnapshot,
     /// Multi-bucket view keyed by metered `limit_id` (for example, `codex`).
     pub rate_limits_by_limit_id: Option<HashMap<String, RateLimitSnapshot>>,
+    pub rate_limit_reset_credits: Option<RateLimitResetCreditsSummary>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct RateLimitResetCreditsSummary {
+    pub available_count: i64,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ConsumeAccountRateLimitResetCreditParams {
+    /// Identifies one logical reset attempt. A UUID is recommended; reuse the same value when
+    /// retrying that attempt.
+    pub idempotency_key: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ConsumeAccountRateLimitResetCreditResponse {
+    pub outcome: ConsumeAccountRateLimitResetCreditOutcome,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/", rename_all = "camelCase")]
+pub enum ConsumeAccountRateLimitResetCreditOutcome {
+    /// A reset credit was consumed and the eligible rate-limit windows were reset.
+    Reset,
+    /// No current rate-limit window is eligible for a reset.
+    NothingToReset,
+    /// The account has no earned reset credits available.
+    NoCredit,
+    /// The same idempotency key already completed a reset successfully.
+    AlreadyRedeemed,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]