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/tests/test_async_client_behavior.py

@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+import asyncio
+import time
+
+from codex_app_server.async_client import AsyncAppServerClient
+
+
+def test_async_client_serializes_transport_calls() -> None:
+    async def scenario() -> int:
+        client = AsyncAppServerClient()
+        active = 0
+        max_active = 0
+
+        def fake_model_list(include_hidden: bool = False) -> bool:
+            nonlocal active, max_active
+            active += 1
+            max_active = max(max_active, active)
+            time.sleep(0.05)
+            active -= 1
+            return include_hidden
+
+        client._sync.model_list = fake_model_list  # type: ignore[method-assign]
+        await asyncio.gather(client.model_list(), client.model_list())
+        return max_active
+
+    assert asyncio.run(scenario()) == 1
+
+
+def test_async_stream_text_is_incremental_and_blocks_parallel_calls() -> None:
+    async def scenario() -> tuple[str, list[str], bool]:
+        client = AsyncAppServerClient()
+
+        def fake_stream_text(thread_id: str, text: str, params=None):  # type: ignore[no-untyped-def]
+            yield "first"
+            time.sleep(0.03)
+            yield "second"
+            yield "third"
+
+        def fake_model_list(include_hidden: bool = False) -> str:
+            return "done"
+
+        client._sync.stream_text = fake_stream_text  # type: ignore[method-assign]
+        client._sync.model_list = fake_model_list  # type: ignore[method-assign]
+
+        stream = client.stream_text("thread-1", "hello")
+        first = await anext(stream)
+
+        blocked_before_stream_done = False
+        competing_call = asyncio.create_task(client.model_list())
+        await asyncio.sleep(0.01)
+        blocked_before_stream_done = not competing_call.done()
+
+        remaining: list[str] = []
+        async for item in stream:
+            remaining.append(item)
+
+        await competing_call
+        return first, remaining, blocked_before_stream_done
+
+    first, remaining, blocked = asyncio.run(scenario())
+    assert first == "first"
+    assert remaining == ["second", "third"]
+    assert blocked