[4/8] Define Python SDK public API surface

[aibrahim-oai]

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

Verification

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

[aibrahim-oai]

This change is part of the following stack:

• Publish Python runtime wheels on release #21812
  • [1/8] Pin Python SDK runtime dependency #21891
    • [2/8] Generate Python SDK types from pinned runtime #21893
      • [3/8] Run Python SDK tests in CI #21895
        • [4/8] Define Python SDK public API surface #21896 ◀

_{Change managed by git-spice.}