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,