[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/getting-started.md

@@ -95,12 +95,13 @@ with Codex() as codex:
     print(result.final_response)
 ```
 
-## 6) Generated models
+## 6) Public app-server types
 
-The convenience wrappers live at the package root, but the canonical app-server models live under:
+The convenience wrappers live at the package root. Public app-server value and
+event types live under:
 
 ```python
-from codex_app_server.generated.v2_all import Turn, TurnStatus, ThreadReadResponse
+from codex_app_server.types import ThreadReadResponse, Turn, TurnStatus
 ```
 
 ## 7) Next stops