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/examples/05_existing_thread/async.py

@@ -0,0 +1,34 @@
+import sys
+from pathlib import Path
+
+_EXAMPLES_ROOT = Path(__file__).resolve().parents[1]
+if str(_EXAMPLES_ROOT) not in sys.path:
+    sys.path.insert(0, str(_EXAMPLES_ROOT))
+
+from _bootstrap import assistant_text_from_turn, ensure_local_sdk_src, find_turn_by_id, runtime_config
+
+ensure_local_sdk_src()
+
+import asyncio
+
+from codex_app_server import AsyncCodex, TextInput
+
+
+async def main() -> None:
+    async with AsyncCodex(config=runtime_config()) as codex:
+        original = await codex.thread_start(model="gpt-5.4", config={"model_reasoning_effort": "high"})
+
+        first_turn = await original.turn(TextInput("Tell me one fact about Saturn."))
+        _ = await first_turn.run()
+        print("Created thread:", original.id)
+
+        resumed = await codex.thread_resume(original.id)
+        second_turn = await resumed.turn(TextInput("Continue with one more fact."))
+        second = await second_turn.run()
+        persisted = await resumed.read(include_turns=True)
+        persisted_turn = find_turn_by_id(persisted.thread.turns, second.id)
+        print(assistant_text_from_turn(persisted_turn))
+
+
+if __name__ == "__main__":
+    asyncio.run(main())