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/README.md

@@ -26,6 +26,8 @@ when you intentionally want to run against a specific local app-server binary.
 from openai_codex import Codex
 
 with Codex() as codex:
+    # Call login_api_key(...) first when this app-server session is not
+    # already authenticated.
     thread = codex.thread_start(model="gpt-5")
     result = thread.run("Say hello in one sentence.")
     print(result.final_response)
@@ -35,6 +37,34 @@ with Codex() as codex:
 `result.final_response` is `None` when the turn completes without a final-answer
 or phase-less assistant message item.
 
+## Login
+
+Use the auth helper that matches your app:
+
+```python
+from openai_codex import Codex
+
+with Codex() as codex:
+    codex.login_api_key("sk-...")
+    account = codex.account()
+    print(account.account)
+```
+
+Interactive ChatGPT login returns a handle. Open the provided URL or device-code
+page, then wait for the matching completion event:
+
+```python
+with Codex() as codex:
+    login = codex.login_chatgpt()
+    print(login.auth_url)
+    completed = login.wait()
+    print(completed.success)
+```
+
+Use `login_chatgpt_device_code()` for device-code auth, `handle.cancel()` to
+stop an in-progress interactive login, and `logout()` to clear the active
+app-server account session.
+
 ## Docs map
 
 - Golden path tutorial: `docs/getting-started.md`