[app-server] Expose rate-limit reset credit details

[jayp-oai]

Why

Supported clients currently receive only the reset-credit count from account/rateLimits/read. The redemption UI needs the available credits, their expiry times, and a way to consume the credit the user selected without accessing private backend APIs directly.

What changed

• add the v2 account/rateLimitResetCredit/list RPC
• return the full availableCount plus up to 10 detail rows, matching the backend response cap
• keep account/rateLimits/read unchanged and count-only
• add an optional nullable creditId to account/rateLimitResetCredit/consume; omitting it preserves the existing automatic selection behavior
• forward a selected credit ID to the Codex backend and map the detail response into the app-server protocol
• update the app-server README and generated schema fixtures

The TUI consumer is stacked in #30488.

Validation

• just test -p codex-app-server-protocol (251 passed)
• just test -p codex-backend-client (16 passed)
• just test -p codex-app-server rate_limit_reset_credit (9 passed)
• just test -p codex-tui chatwidget::tests::usage (29 passed compatibility check)
• just fix -p codex-backend-client -p codex-app-server-protocol -p codex-app-server -p codex-tui
• just fmt
• Autoreview: clean

Part of #29618.