Add Python app-server SDK (#14435)

## TL;DR
Bring the Python app-server SDK from `main-with-prs-13953-and-14232`
onto current `main` as a standalone SDK-only PR.

- adds the new `sdk/python` and `sdk/python-runtime` package trees
- keeps the scope to the SDK payload only, without the unrelated
branch-history or workflow changes from the source branch
- regenerates `sdk/python/src/codex_app_server/generated/v2_all.py`
against current `main` schema so the extracted SDK matches today's
protocol definitions

## Validation
- `PYTHONPATH=sdk/python/src python3 -m pytest sdk/python/tests`

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

sdk/python/src/codex_app_server/models.py

@@ -0,0 +1,97 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TypeAlias
+
+from pydantic import BaseModel
+
+from .generated.v2_all import (
+    AccountLoginCompletedNotification,
+    AccountRateLimitsUpdatedNotification,
+    AccountUpdatedNotification,
+    AgentMessageDeltaNotification,
+    AppListUpdatedNotification,
+    CommandExecutionOutputDeltaNotification,
+    ConfigWarningNotification,
+    ContextCompactedNotification,
+    DeprecationNoticeNotification,
+    ErrorNotification,
+    FileChangeOutputDeltaNotification,
+    ItemCompletedNotification,
+    ItemStartedNotification,
+    McpServerOauthLoginCompletedNotification,
+    McpToolCallProgressNotification,
+    PlanDeltaNotification,
+    RawResponseItemCompletedNotification,
+    ReasoningSummaryPartAddedNotification,
+    ReasoningSummaryTextDeltaNotification,
+    ReasoningTextDeltaNotification,
+    TerminalInteractionNotification,
+    ThreadNameUpdatedNotification,
+    ThreadStartedNotification,
+    ThreadTokenUsageUpdatedNotification,
+    TurnCompletedNotification,
+    TurnDiffUpdatedNotification,
+    TurnPlanUpdatedNotification,
+    TurnStartedNotification,
+    WindowsWorldWritableWarningNotification,
+)
+
+JsonScalar: TypeAlias = str | int | float | bool | None
+JsonValue: TypeAlias = JsonScalar | dict[str, "JsonValue"] | list["JsonValue"]
+JsonObject: TypeAlias = dict[str, JsonValue]
+
+
+@dataclass(slots=True)
+class UnknownNotification:
+    params: JsonObject
+
+
+NotificationPayload: TypeAlias = (
+    AccountLoginCompletedNotification
+    | AccountRateLimitsUpdatedNotification
+    | AccountUpdatedNotification
+    | AgentMessageDeltaNotification
+    | AppListUpdatedNotification
+    | CommandExecutionOutputDeltaNotification
+    | ConfigWarningNotification
+    | ContextCompactedNotification
+    | DeprecationNoticeNotification
+    | ErrorNotification
+    | FileChangeOutputDeltaNotification
+    | ItemCompletedNotification
+    | ItemStartedNotification
+    | McpServerOauthLoginCompletedNotification
+    | McpToolCallProgressNotification
+    | PlanDeltaNotification
+    | RawResponseItemCompletedNotification
+    | ReasoningSummaryPartAddedNotification
+    | ReasoningSummaryTextDeltaNotification
+    | ReasoningTextDeltaNotification
+    | TerminalInteractionNotification
+    | ThreadNameUpdatedNotification
+    | ThreadStartedNotification
+    | ThreadTokenUsageUpdatedNotification
+    | TurnCompletedNotification
+    | TurnDiffUpdatedNotification
+    | TurnPlanUpdatedNotification
+    | TurnStartedNotification
+    | WindowsWorldWritableWarningNotification
+    | UnknownNotification
+)
+
+
+@dataclass(slots=True)
+class Notification:
+    method: str
+    payload: NotificationPayload
+
+
+class ServerInfo(BaseModel):
+    name: str | None = None
+    version: str | None = None
+
+
+class InitializeResponse(BaseModel):
+    serverInfo: ServerInfo | None = None
+    userAgent: str | None = None