Add untrusted external messages to the Python SDK (#44086)

## Why

Applications need to deliver content from other agents, tools, or services with tool-level authority, without treating it as user input or granting authorization.

## What changed

- Export `ExternalMessage` for sync and async `run(...)` and `turn(...)`, accepting text or structured content with a tool name and optional namespace. Send it through `toolOutput` and require CLI 0.151.0 or newer.
- Support starting a turn or joining an active regular turn while preserving external content as function output in history. Keep external messages separate from user-input lists and `steer(...)`.
- Give turn handles independent subscriptions, replaying completed items and latest usage to joining handles. Release consumed transient events and clean up subscriptions on closure, failure, or cancellation.
- Document the authority boundary and add sync and async examples.

## Testing

Add coverage for wire representations, input validation, runtime compatibility, tool authority across resume, active-turn joins, and tool-output truncation. Add subscription tests for replay, concurrent consumers, early completion, cancellation, and cleanup.

GitOrigin-RevId: 6106327085fd9c4bd11b71e20b3d8e74738b8bb5

sdk/python/src/openai_codex/__init__.py

@@ -23,6 +23,7 @@ from .api import (
     ChatgptLoginHandle,
     Codex,
     DeviceCodeLoginHandle,
+    ExternalMessage,
     ImageInput,
     Input,
     InputItem,
@@ -72,6 +73,7 @@ __all__ = [
     "Input",
     "InputItem",
     "RunInput",
+    "ExternalMessage",
     "TextInput",
     "ImageInput",
     "LocalImageInput",