Add Python SDK public API and examples (#14446)

## TL;DR
WIP esp the examples

Thin the Python SDK public surface so the wrapper layer returns
canonical app-server generated models directly.

- keeps `Codex` / `AsyncCodex` / `Thread` / `Turn` and input helpers,
but removes alias-only type layers and custom result models
- `metadata` now returns `InitializeResponse` and `run()` returns the
generated app-server `Turn`
- updates docs, examples, notebook, and tests to use canonical generated
types and regenerates `v2_all.py` against current schema
- keeps the pinned runtime-package integration flow and real integration
coverage

  ## Validation
  - `PYTHONPATH=sdk/python/src python3 -m pytest sdk/python/tests`
- `GH_TOKEN="$(gh auth token)" RUN_REAL_CODEX_TESTS=1
PYTHONPATH=sdk/python/src python3 -m pytest sdk/python/tests -rs`

---------

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

sdk/python/src/codex_app_server/generated/v2_types.py

@@ -1,25 +0,0 @@
-"""Stable aliases over full v2 autogenerated models (datamodel-code-generator)."""
-
-from .v2_all.ModelListResponse import ModelListResponse
-from .v2_all.ThreadCompactStartResponse import ThreadCompactStartResponse
-from .v2_all.ThreadListResponse import ThreadListResponse
-from .v2_all.ThreadReadResponse import ThreadReadResponse
-from .v2_all.ThreadTokenUsageUpdatedNotification import (
-    ThreadTokenUsageUpdatedNotification,
-)
-from .v2_all.TurnCompletedNotification import ThreadItem153 as ThreadItem
-from .v2_all.TurnCompletedNotification import (
-    TurnCompletedNotification as TurnCompletedNotificationPayload,
-)
-from .v2_all.TurnSteerResponse import TurnSteerResponse
-
-__all__ = [
-    "ModelListResponse",
-    "ThreadCompactStartResponse",
-    "ThreadListResponse",
-    "ThreadReadResponse",
-    "ThreadTokenUsageUpdatedNotification",
-    "TurnCompletedNotificationPayload",
-    "TurnSteerResponse",
-    "ThreadItem",
-]