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/getting-started.md

@@ -19,9 +19,37 @@ Requirements:
 - Python `>=3.10`
 - uv
 - installed `openai-codex-cli-bin` runtime package, or an explicit `codex_bin` override
-- local Codex auth/session configured
 
-## 2) Run your first turn (sync)
+## 2) Authenticate when needed
+
+Existing Codex auth state is reused automatically. To authenticate from the SDK,
+use the flow that fits 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 browser login returns a handle that carries the URL and the
+matching completion event:
+
+```python
+with Codex() as codex:
+    login = codex.login_chatgpt()
+    print(login.auth_url)
+    completed = login.wait()
+    print(completed.success)
+```
+
+Device-code login works the same way with
+`login_chatgpt_device_code()`, which exposes `verification_url`, `user_code`,
+and `wait()`.
+
+## 3) Run your first turn (sync)
 
 ```python
 from openai_codex import Codex
@@ -47,7 +75,7 @@ What happened:
 - use `thread.turn(...)` when you need a `TurnHandle` for streaming, steering, interrupting, or turn IDs/status
 - one client can consume multiple active turns concurrently; turn streams are routed by turn ID
 
-## 3) Continue the same thread (multi-turn)
+## 4) Continue the same thread (multi-turn)
 
 ```python
 from openai_codex import Codex
@@ -62,7 +90,7 @@ with Codex() as codex:
     print("second:", second.final_response)
 ```
 
-## 4) Async parity
+## 5) Async parity
 
 Use `async with AsyncCodex()` as the normal async entrypoint. `AsyncCodex`
 initializes lazily, and context entry makes startup/shutdown explicit.
@@ -82,7 +110,7 @@ async def main() -> None:
 asyncio.run(main())
 ```
 
-## 5) Resume an existing thread
+## 6) Resume an existing thread
 
 ```python
 from openai_codex import Codex
@@ -95,7 +123,7 @@ with Codex() as codex:
     print(result.final_response)
 ```
 
-## 6) Public app-server types
+## 7) Public app-server types
 
 The convenience wrappers live at the package root. Public app-server value and
 event types live under:
@@ -104,7 +132,7 @@ event types live under:
 from openai_codex.types import ThreadReadResponse, Turn, TurnStatus
 ```
 
-## 7) Next stops
+## 8) Next stops
 
 - API surface and signatures: `docs/api-reference.md`
 - Common decisions/pitfalls: `docs/faq.md`