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/src/openai_codex/__init__.py

@@ -1,10 +1,14 @@
 from ._version import __version__
 from .api import (
     ApprovalMode,
+    AsyncChatgptLoginHandle,
     AsyncCodex,
+    AsyncDeviceCodeLoginHandle,
     AsyncThread,
     AsyncTurnHandle,
+    ChatgptLoginHandle,
     Codex,
+    DeviceCodeLoginHandle,
     ImageInput,
     Input,
     InputItem,
@@ -39,6 +43,10 @@ __all__ = [
     "Codex",
     "AsyncCodex",
     "ApprovalMode",
+    "ChatgptLoginHandle",
+    "DeviceCodeLoginHandle",
+    "AsyncChatgptLoginHandle",
+    "AsyncDeviceCodeLoginHandle",
     "Thread",
     "AsyncThread",
     "TurnHandle",