[4/8] Define Python SDK public API surface (#21896)

## Why

The SDK package root should be the ergonomic public client API, not a
dump of every generated app-server schema type. Generated models still
need a supported import path, but callers should be able to tell which
names are high-level SDK entrypoints and which names are protocol value
models.

## What

- Define a curated root `__all__` for clients, handles, input helpers,
retry helpers, config, and public errors.
- Add a `types` module as the supported home for generated app-server
response, event, enum, and helper models.
- Update docs and examples to import protocol/value models from the type
module.
- Add tests that lock root exports, type-module exports, star-import
behavior, and example import hygiene.

## Stack

1. #21891 `[1/8]` Pin Python SDK runtime dependency
2. #21893 `[2/8]` Generate Python SDK types from pinned runtime
3. #21895 `[3/8]` Run Python SDK tests in CI
4. This PR `[4/8]` Define Python SDK public API surface
5. #21905 `[5/8]` Rename Python SDK package to `openai-codex`
6. #21910 `[6/8]` Add high-level Python SDK approval mode
7. #22014 `[7/8]` Add Python SDK app-server integration harness
8. #22021 `[8/8]` Add Python SDK Ruff formatting

## Verification

- Added public API signature tests for root exports, `types` exports,
and example imports.

---------

Co-authored-by: Codex <noreply@openai.com>

sdk/python/docs/faq.md

@@ -8,7 +8,7 @@
 
 ## `run()` vs `stream()`
 
-- `TurnHandle.run()` / `AsyncTurnHandle.run()` is the easiest path. It consumes events until completion and returns the canonical generated app-server `Turn` model.
+- `TurnHandle.run()` / `AsyncTurnHandle.run()` is the easiest path. It consumes events until completion and returns the public app-server `Turn` model from `codex_app_server.types`.
 - `TurnHandle.stream()` / `AsyncTurnHandle.stream()` yields raw notifications (`Notification`) so you can react event-by-event.
 
 Choose `run()` for most apps. Choose `stream()` for progress UIs, custom timeout logic, or custom parsing.
@@ -99,5 +99,5 @@ Do not blindly retry all errors. For `InvalidParamsError` or `MethodNotFoundErro
 
 - Starting a new thread for every prompt when you wanted continuity.
 - Forgetting to `close()` (or not using context managers).
-- Assuming `run()` returns extra SDK-only fields instead of the generated `Turn` model.
+- Assuming `run()` returns extra SDK-only fields instead of the public `Turn` model.
 - Mixing SDK input classes with raw dicts incorrectly.