feat(app-server): expose account token usage [1 of 2] (#25344)

## Why

Token activity is useful account-level context, but terminal clients
need a supported app-server path to fetch it without reaching into
ChatGPT backend details directly. The API should also live under the
broader account usage umbrella so future usage surfaces can be added
without proliferating user-facing concepts.

## What Changed

- Add `codex-backend-client` support for the ChatGPT profile token-usage
payload.
- Add the v2 `account/usage/read` app-server RPC.
- Map lifetime usage, peak daily usage, streak, longest task duration,
and daily buckets into app-server protocol types.
- Gate the request on Codex-backend auth, which supports ChatGPT auth
tokens and AgentIdentity.
- Regenerate the app-server JSON and TypeScript schema fixtures.

## Token Count Source

`account/usage/read` returns the token-usage aggregate supplied by the
ChatGPT profile backend. App-server maps that backend-owned aggregate
into protocol fields; it does not recompute cached-token treatment,
usage multipliers, or raw input/output totals locally.

## Stack

1. feat(app-server): expose account token usage [1 of 2] (this PR)
2. [#25345](https://github.com/openai/codex/pull/25345) feat(tui): add
token activity command [2 of 2]

## How to Test

1. Start an app-server client from this branch while authenticated with
ChatGPT or AgentIdentity.
2. Call `account/usage/read`.
3. Confirm the response includes `summary` and `dailyUsageBuckets`.
4. Also verify a session without Codex-backend auth receives the
existing auth error path.

Targeted tests:
- `just test -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server`
- `just write-app-server-schema`

codex-rs/app-server/src/message_processor.rs

@@ -1294,6 +1294,9 @@ impl MessageProcessor {
             ClientRequest::GetAccountRateLimits { .. } => {
                 self.account_processor.get_account_rate_limits().await
             }
+            ClientRequest::GetAccountTokenUsage { .. } => {
+                self.account_processor.get_account_token_usage().await
+            }
             ClientRequest::SendAddCreditsNudgeEmail { params, .. } => {
                 self.account_processor
                     .send_add_credits_nudge_email(params)