python-sdk: generated type foundation (all v2 schemas) (#13953)

## Summary
Foundation PR only (base for PR #3).

This PR contains the SDK runtime foundation and generated artifacts:

- pinned runtime binary in `sdk/python/bin/` (`codex` or `codex.exe` by
platform)
- single maintenance script:
`sdk/python/scripts/update_sdk_artifacts.py`
- generated protocol/types artifacts under:
  - `sdk/python/src/codex_app_server/generated/protocol_types.py`
  - `sdk/python/src/codex_app_server/generated/schema_types.py`
  - `sdk/python/src/codex_app_server/generated/v2_all/*`
- generation-contract test wiring (`tests/test_contract_generation.py`)

## Release asset behavior
`update_sdk_artifacts.py` now:
- selects latest release by channel (`--channel stable|alpha`)
- resolves the correct asset for current OS/arch
- extracts platform binary (`codex` on macOS/Linux, `codex.exe` on
Windows)
- keeps runtime on single pinned binary source in `sdk/python/bin/`

## Scope boundary
- ✅ PR #2 = binary + generation pipeline + generated types foundation
- ❌ PR #2 does **not** include examples/integration logic polish (that
is PR #3)

## Validation
- Ran: `python scripts/update_sdk_artifacts.py --channel stable`
- Regenerated and committed resulting generated artifacts
- Local tests pass on branch

sdk/python/tests/test_client_rpc_methods.py

@@ -0,0 +1,95 @@
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from codex_app_server.client import AppServerClient, _params_dict
+from codex_app_server.generated.v2_all import ThreadListParams, ThreadTokenUsageUpdatedNotification
+from codex_app_server.models import UnknownNotification
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+def test_thread_set_name_and_compact_use_current_rpc_methods() -> None:
+    client = AppServerClient()
+    calls: list[tuple[str, dict[str, Any] | None]] = []
+
+    def fake_request(method: str, params, *, response_model):  # type: ignore[no-untyped-def]
+        calls.append((method, params))
+        return response_model.model_validate({})
+
+    client.request = fake_request  # type: ignore[method-assign]
+
+    client.thread_set_name("thread-1", "sdk-name")
+    client.thread_compact("thread-1")
+
+    assert calls[0][0] == "thread/name/set"
+    assert calls[1][0] == "thread/compact/start"
+
+
+def test_generated_params_models_are_snake_case_and_dump_by_alias() -> None:
+    params = ThreadListParams(search_term="needle", limit=5)
+
+    assert "search_term" in ThreadListParams.model_fields
+    dumped = _params_dict(params)
+    assert dumped == {"searchTerm": "needle", "limit": 5}
+
+
+def test_generated_v2_bundle_has_single_shared_plan_type_definition() -> None:
+    source = (ROOT / "src" / "codex_app_server" / "generated" / "v2_all.py").read_text()
+    assert source.count("class PlanType(") == 1
+
+
+def test_notifications_are_typed_with_canonical_v2_methods() -> None:
+    client = AppServerClient()
+    event = client._coerce_notification(
+        "thread/tokenUsage/updated",
+        {
+            "threadId": "thread-1",
+            "turnId": "turn-1",
+            "tokenUsage": {
+                "last": {
+                    "cachedInputTokens": 0,
+                    "inputTokens": 1,
+                    "outputTokens": 2,
+                    "reasoningOutputTokens": 0,
+                    "totalTokens": 3,
+                },
+                "total": {
+                    "cachedInputTokens": 0,
+                    "inputTokens": 1,
+                    "outputTokens": 2,
+                    "reasoningOutputTokens": 0,
+                    "totalTokens": 3,
+                },
+            },
+        },
+    )
+
+    assert event.method == "thread/tokenUsage/updated"
+    assert isinstance(event.payload, ThreadTokenUsageUpdatedNotification)
+    assert event.payload.turn_id == "turn-1"
+
+
+def test_unknown_notifications_fall_back_to_unknown_payloads() -> None:
+    client = AppServerClient()
+    event = client._coerce_notification(
+        "unknown/notification",
+        {
+            "id": "evt-1",
+            "conversationId": "thread-1",
+            "msg": {"type": "turn_aborted"},
+        },
+    )
+
+    assert event.method == "unknown/notification"
+    assert isinstance(event.payload, UnknownNotification)
+    assert event.payload.params["msg"] == {"type": "turn_aborted"}
+
+
+def test_invalid_notification_payload_falls_back_to_unknown() -> None:
+    client = AppServerClient()
+    event = client._coerce_notification("thread/tokenUsage/updated", {"threadId": "missing"})
+
+    assert event.method == "thread/tokenUsage/updated"
+    assert isinstance(event.payload, UnknownNotification)