sdk/python: add first-class login support (#23093)

## Why

The Python SDK can already create threads and run turns, but
authentication still has to be arranged outside the SDK. App-server
already exposes account login, account inspection, logout, and
`account/login/completed` notifications, so SDK users currently have to
work around a missing public client layer for a core setup step.

This change makes authentication a normal SDK workflow while preserving
the backend flow shape: API-key login completes immediately, and
interactive ChatGPT flows return live handles that complete later
through app-server notifications.

## What changed

- Added public sync and async auth methods on `Codex` / `AsyncCodex`:
  - `login_api_key(...)`
  - `login_chatgpt()`
  - `login_chatgpt_device_code()`
  - `account(...)`
  - `logout()`
- Added public browser-login and device-code handle types with
attempt-local `wait()` and `cancel()` helpers. Cancellation stays on the
handle instead of a root-level SDK method.
- Extended the Python app-server client and notification router so login
completion events are routed by `login_id` without consuming unrelated
global notifications.
- Kept login request/handle logic in a focused internal `_login.py`
module so `api.py` remains the public facade instead of absorbing more
auth plumbing.
- Exported the new handle types plus curated account/login response
types from the SDK surfaces.
- Updated SDK docs, added sync/async login walkthrough examples, and
added a notebook login walkthrough cell.

## Verification

Added SDK coverage for:

- API-key login, account readback, and logout through the app-server
harness in both sync and async clients.
- Browser login cancellation plus `handle.wait()` completion through the
real app-server boundary used by the Python SDK harness.
- Waiter routing that stays scoped across replaced interactive login
attempts, plus async handle cancellation coverage.
- Login notification demuxing, replay of early completion events, and
async client delegation.
- Public export/signature assertions.
- Real integration-suite smoke coverage for the new examples and
notebook login cell.

sdk/python/docs/faq.md

@@ -23,6 +23,16 @@ Choose `run()` for most apps. Choose `stream()` for progress UIs, custom timeout
 
 If your app is not already async, stay with `Codex`.
 
+## How do I log in?
+
+- `login_api_key(...)` authenticates immediately with an API key.
+- `login_chatgpt()` starts browser login and returns a handle with `auth_url`.
+- `login_chatgpt_device_code()` starts device-code login and returns a handle
+  with `verification_url` and `user_code`.
+- Interactive handles expose `wait()` for the matching
+  `account/login/completed` notification and `cancel()` to stop that attempt.
+- `account()` reads the current account state, and `logout()` clears it.
+
 ## Public kwargs are snake_case
 
 Public API keyword names are snake_case. The SDK still maps them to wire camelCase under the hood.
@@ -56,7 +66,6 @@ Common causes:
 
 - published runtime package (`openai-codex-cli-bin`) is not installed
 - local `codex_bin` override points to a missing file
-- local auth/session is missing
 - incompatible/old app-server
 
 ## Why does a turn "hang"?