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/api-reference.md

@@ -12,6 +12,10 @@ from openai_codex import (
     Codex,
     AsyncCodex,
     ApprovalMode,
+    ChatgptLoginHandle,
+    DeviceCodeLoginHandle,
+    AsyncChatgptLoginHandle,
+    AsyncDeviceCodeLoginHandle,
     RunResult,
     Thread,
     AsyncThread,
@@ -26,6 +30,11 @@ from openai_codex import (
     MentionInput,
 )
 from openai_codex.types import (
+    Account,
+    AccountLoginCompletedNotification,
+    CancelLoginAccountResponse,
+    CancelLoginAccountStatus,
+    GetAccountResponse,
     InitializeResponse,
     ThreadItem,
     ThreadTokenUsage,
@@ -47,6 +56,11 @@ Properties/methods:
 
 - `metadata -> InitializeResponse`
 - `close() -> None`
+- `login_api_key(api_key: str) -> None`
+- `login_chatgpt() -> ChatgptLoginHandle`
+- `login_chatgpt_device_code() -> DeviceCodeLoginHandle`
+- `account(*, refresh_token: bool = False) -> GetAccountResponse`
+- `logout() -> None`
 - `thread_start(*, approval_mode=ApprovalMode.auto_review, base_instructions=None, config=None, cwd=None, developer_instructions=None, ephemeral=None, model=None, model_provider=None, personality=None, sandbox=None) -> Thread`
 - `thread_list(*, archived=None, cursor=None, cwd=None, limit=None, model_providers=None, sort_key=None, source_kinds=None) -> ThreadListResponse`
 - `thread_resume(thread_id: str, *, approval_mode=ApprovalMode.auto_review, base_instructions=None, config=None, cwd=None, developer_instructions=None, model=None, model_provider=None, personality=None, sandbox=None) -> Thread`
@@ -82,6 +96,11 @@ Properties/methods:
 
 - `metadata -> InitializeResponse`
 - `close() -> Awaitable[None]`
+- `login_api_key(api_key: str) -> Awaitable[None]`
+- `login_chatgpt() -> Awaitable[AsyncChatgptLoginHandle]`
+- `login_chatgpt_device_code() -> Awaitable[AsyncDeviceCodeLoginHandle]`
+- `account(*, refresh_token: bool = False) -> Awaitable[GetAccountResponse]`
+- `logout() -> Awaitable[None]`
 - `thread_start(*, approval_mode=ApprovalMode.auto_review, base_instructions=None, config=None, cwd=None, developer_instructions=None, ephemeral=None, model=None, model_provider=None, personality=None, sandbox=None) -> Awaitable[AsyncThread]`
 - `thread_list(*, archived=None, cursor=None, cwd=None, limit=None, model_providers=None, sort_key=None, source_kinds=None) -> Awaitable[ThreadListResponse]`
 - `thread_resume(thread_id: str, *, approval_mode=ApprovalMode.auto_review, base_instructions=None, config=None, cwd=None, developer_instructions=None, model=None, model_provider=None, personality=None, sandbox=None) -> Awaitable[AsyncThread]`
@@ -97,6 +116,30 @@ async with AsyncCodex() as codex:
     ...
 ```
 
+## Login handles
+
+### ChatgptLoginHandle / AsyncChatgptLoginHandle
+
+- `login_id: str`
+- `auth_url: str`
+- `wait() -> AccountLoginCompletedNotification`
+- `cancel() -> CancelLoginAccountResponse`
+
+Async handle methods return awaitables.
+
+### DeviceCodeLoginHandle / AsyncDeviceCodeLoginHandle
+
+- `login_id: str`
+- `verification_url: str`
+- `user_code: str`
+- `wait() -> AccountLoginCompletedNotification`
+- `cancel() -> CancelLoginAccountResponse`
+
+Async handle methods return awaitables.
+
+`wait()` consumes only the completion notification for its matching login
+attempt. API-key login completes synchronously and does not return a handle.
+
 ## Thread / AsyncThread
 
 `Thread` and `AsyncThread` share the same shape and intent.
@@ -176,6 +219,11 @@ The SDK wrappers return and accept public app-server models wherever possible:
 
 ```python
 from openai_codex.types import (
+    Account,
+    AccountLoginCompletedNotification,
+    CancelLoginAccountResponse,
+    CancelLoginAccountStatus,
+    GetAccountResponse,
     ThreadReadResponse,
     Turn,
     TurnStatus,