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/src/message_processor.rs

@@ -1376,6 +1376,11 @@ impl MessageProcessor {
             ClientRequest::GetAccountRateLimits { .. } => {
                 self.account_processor.get_account_rate_limits().await
             }
+            ClientRequest::ConsumeAccountRateLimitResetCredit { params, .. } => {
+                self.account_processor
+                    .consume_account_rate_limit_reset_credit(params)
+                    .await
+            }
             ClientRequest::GetAccountTokenUsage { .. } => {
                 self.account_processor.get_account_token_usage().await
             }