Add ChatGPT device-code login to app server (#15525)

## Problem

App-server clients could only initiate ChatGPT login through the browser
callback flow, even though the shared login crate already supports
device-code auth. That left VS Code, Codex App, and other app-server
clients without a first-class way to use the existing device-code
backend when browser redirects are brittle or when the client UX wants
to own the login ceremony.

## Mental model

This change adds a second ChatGPT login start path to app-server:
clients can now call `account/login/start` with `type:
"chatgptDeviceCode"`. App-server immediately returns a `loginId` plus
the device-code UX payload (`verificationUrl` and `userCode`), then
completes the login asynchronously in the background using the existing
`codex_login` polling flow. Successful device-code login still resolves
to ordinary `chatgpt` auth, and completion continues to flow through the
existing `account/login/completed` and `account/updated` notifications.

## Non-goals

This does not introduce a new auth mode, a new account shape, or a
device-code eligibility discovery API. It also does not add automatic
fallback to browser login in core; clients remain responsible for
choosing when to request device code and whether to retry with a
different UX if the backend/admin policy rejects it.

## Tradeoffs

We intentionally keep `login_chatgpt_common` as a local validation
helper instead of turning it into a capability probe. Device-code
eligibility is checked by actually calling `request_device_code`, which
means policy-disabled cases surface as an immediate request error rather
than an async completion event. We also keep the active-login state
machine minimal: browser and device-code logins share the same public
cancel contract, but device-code cancellation is implemented with a
local cancel token rather than a larger cross-crate refactor.

## Architecture

The protocol grows a new `chatgptDeviceCode` request/response variant in
app-server v2. On the server side, the new handler reuses the existing
ChatGPT login precondition checks, calls `request_device_code`, returns
the device-code payload, and then spawns a background task that waits on
either cancellation or `complete_device_code_login`. On success, it
reuses the existing auth reload and cloud-requirements refresh path
before emitting `account/login/completed` success and `account/updated`.
On failure or cancellation, it emits only `account/login/completed`
failure. The existing `account/login/cancel { loginId }` contract
remains unchanged and now works for both browser and device-code
attempts.


## Tests

Added protocol serialization coverage for the new request/response
variant, plus app-server tests for device-code success, failure, cancel,
and start-time rejection behavior. Existing browser ChatGPT login
coverage remains in place to show that the callback-based flow is
unchanged.

codex-rs/app-server/README.md

@@ -1309,14 +1309,14 @@ The JSON-RPC auth/account surface exposes request/response methods plus server-i
 Codex supports these authentication modes. The current mode is surfaced in `account/updated` (`authMode`), which also includes the current ChatGPT `planType` when available, and can be inferred from `account/read`.
 
 - **API key (`apiKey`)**: Caller supplies an OpenAI API key via `account/login/start` with `type: "apiKey"`. The API key is saved and used for API requests.
-- **ChatGPT managed (`chatgpt`)** (recommended): Codex owns the ChatGPT OAuth flow and refresh tokens. Start via `account/login/start` with `type: "chatgpt"`; Codex persists tokens to disk and refreshes them automatically.
+- **ChatGPT managed (`chatgpt`)** (recommended): Codex owns the ChatGPT OAuth flow and refresh tokens. Start via `account/login/start` with `type: "chatgpt"` for the browser flow or `type: "chatgptDeviceCode"` for device code; Codex persists tokens to disk and refreshes them automatically.
 
 ### API Overview
 
 - `account/read` — fetch current account info; optionally refresh tokens.
-- `account/login/start` — begin login (`apiKey`, `chatgpt`).
+- `account/login/start` — begin login (`apiKey`, `chatgpt`, `chatgptDeviceCode`).
 - `account/login/completed` (notify) — emitted when a login attempt finishes (success or error).
-- `account/login/cancel` — cancel a pending ChatGPT login by `loginId`.
+- `account/login/cancel` — cancel a pending managed ChatGPT login by `loginId`.
 - `account/logout` — sign out; triggers `account/updated`.
 - `account/updated` (notify) — emitted whenever auth mode changes (`authMode`: `apikey`, `chatgpt`, or `null`) and includes the current ChatGPT `planType` when available.
 - `account/rateLimits/read` — fetch ChatGPT rate limits; updates arrive via `account/rateLimits/updated` (notify).
@@ -1380,26 +1380,40 @@ Field notes:
    { "method": "account/updated", "params": { "authMode": "chatgpt", "planType": "plus" } }
    ```
 
-### 4) Cancel a ChatGPT login
+### 4) Log in with ChatGPT (device code flow)
+
+1. Start:
+   ```json
+   { "method": "account/login/start", "id": 4, "params": { "type": "chatgptDeviceCode" } }
+   { "id": 4, "result": { "type": "chatgptDeviceCode", "loginId": "<uuid>", "verificationUrl": "https://auth.openai.com/codex/device", "userCode": "ABCD-1234" } }
+   ```
+2. Show `verificationUrl` and `userCode` to the user; the frontend owns the UX.
+3. Wait for notifications:
+   ```json
+   { "method": "account/login/completed", "params": { "loginId": "<uuid>", "success": true, "error": null } }
+   { "method": "account/updated", "params": { "authMode": "chatgpt", "planType": "plus" } }
+   ```
+
+### 5) Cancel a ChatGPT login
 
 ```json
-{ "method": "account/login/cancel", "id": 4, "params": { "loginId": "<uuid>" } }
+{ "method": "account/login/cancel", "id": 5, "params": { "loginId": "<uuid>" } }
 { "method": "account/login/completed", "params": { "loginId": "<uuid>", "success": false, "error": "…" } }
 ```
 
-### 5) Logout
+### 6) Logout
 
 ```json
-{ "method": "account/logout", "id": 5 }
-{ "id": 5, "result": {} }
+{ "method": "account/logout", "id": 6 }
+{ "id": 6, "result": {} }
 { "method": "account/updated", "params": { "authMode": null, "planType": null } }
 ```
 
-### 6) Rate limits (ChatGPT)
+### 7) Rate limits (ChatGPT)
 
 ```json
-{ "method": "account/rateLimits/read", "id": 6 }
-{ "id": 6, "result": { "rateLimits": { "primary": { "usedPercent": 25, "windowDurationMins": 15, "resetsAt": 1730947200 }, "secondary": null } } }
+{ "method": "account/rateLimits/read", "id": 7 }
+{ "id": 7, "result": { "rateLimits": { "primary": { "usedPercent": 25, "windowDurationMins": 15, "resetsAt": 1730947200 }, "secondary": null } } }
 { "method": "account/rateLimits/updated", "params": { "rateLimits": { … } } }
 ```