[4/6] Define Python SDK public API surface

[aibrahim-oai]

Stack

• [1/6] [1/6] Pin Python SDK runtime dependency #21891 Pin Python SDK runtime dependency
• [2/6] [2/6] Generate Python SDK types from pinned runtime #21893 Generate Python SDK types from pinned runtime
• [3/6] [3/6] Run Python SDK tests in CI #21895 Run Python SDK tests in CI
• [4/6] [4/6] Define Python SDK public API surface #21896 Define Python SDK public API surface
• [5/6] [5/6] Rename Python SDK package to openai-codex #21905 Rename Python SDK package to openai-codex
• [6/6] [6/6] Default Python SDK approval policy to never #21910 Default Python SDK approval policy to never

This PR is the fourth change in the stack, on top of #21895.

Why

The Python SDK package root should stay small and ergonomic. Generated app-server schema objects are still useful for typing, response handling, and notification matching, but exporting them from the root makes it unclear which names are SDK entrypoints and which names are wire/value models.

Python cannot hide shipped submodules, so this PR makes the public import contract explicit: root exports are the client-facing SDK API, and codex_app_server.types is the supported home for app-server value and event models.

What changed

• Shrink codex_app_server.__all__ to config, clients/handles, input helpers, retry helpers, and public errors.
• Add codex_app_server.types for supported app-server response, event, enum, and helper model exports.
• Update SDK docs and runnable examples to import app-server types from codex_app_server.types.
• Extend public API tests to lock root exports, type-module exports, star-import behavior, and examples’ public import surface.

Verification

• git diff --check
• CI is running on this PR.

[aibrahim-oai]

This change is part of the following stack:

• Publish Python runtime wheels on release #21812
  • [1/6] Pin Python SDK runtime dependency #21891
    • [2/6] Generate Python SDK types from pinned runtime #21893
      • [3/6] Run Python SDK tests in CI #21895
        • [4/6] Define Python SDK public API surface #21896 ◀

_{Change managed by git-spice.}