[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/README.md

@@ -5,6 +5,8 @@ Experimental Python SDK for `codex app-server` JSON-RPC v2 over stdio, with a sm
 The generated wire-model layer is sourced from the pinned `openai-codex-cli-bin`
 runtime package and exposed as Pydantic models with snake_case Python fields
 that serialize back to the app-server’s camelCase wire format.
+The package root exports the ergonomic client API; public app-server value and
+event types live in `codex_app_server.types`.
 
 ## Install
 
@@ -110,4 +112,4 @@ This supports the CI release flow:
 - Use context managers (`with Codex() as codex:`) to ensure shutdown.
 - Prefer `thread.run("...")` for the common case. Use `thread.turn(...)` when
   you need streaming, steering, or interrupt control.
-- For transient overload, use `codex_app_server.retry.retry_on_overload`.
+- For transient overload, use `retry_on_overload` from the package root.