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/_inputs.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 
+from collections.abc import Sequence
 from dataclasses import dataclass
 
+from .generated.v2_all import FunctionCallOutputContentItem, TurnToolOutput
 from .models import JsonObject
 
 
@@ -42,9 +44,27 @@ class MentionInput:
     path: str
 
 
+@dataclass(slots=True)
+class ExternalMessage:
+    """Untrusted content supplied by another agent, tool, or application.
+
+    Content has tool-level authority, below user and developer instructions. It
+    does not establish user authorization or approval. Pass this as the whole
+    input to ``thread.run()`` or ``thread.turn()`` to start a turn or join an
+    active regular turn. ``tool_name`` identifies the tool delivering it.
+
+    ``content`` accepts text or Responses-compatible function-output content
+    items. Structured items can be dictionaries; no generated wrapper is needed.
+    """
+
+    tool_name: str
+    content: str | Sequence[JsonObject | FunctionCallOutputContentItem]
+    namespace: str | None = None
+
+
 InputItem = TextInput | ImageInput | LocalImageInput | SkillInput | MentionInput
 Input = list[InputItem] | InputItem
-RunInput = Input | str
+RunInput = Input | str | ExternalMessage
 
 
 def _to_wire_item(item: InputItem) -> JsonObject:
@@ -67,7 +87,17 @@ def _to_wire_input(input: Input) -> list[JsonObject]:
     return [_to_wire_item(input)]
 
 
-def _normalize_run_input(input: RunInput) -> Input:
+def _normalize_run_input(input: Input | str) -> Input:
     if isinstance(input, str):
         return TextInput(input)
     return input
+
+
+def _to_wire_turn_input(input: RunInput) -> tuple[list[JsonObject], TurnToolOutput | None]:
+    if isinstance(input, ExternalMessage):
+        if not isinstance(input.tool_name, str) or not input.tool_name.strip():
+            raise ValueError("ExternalMessage.tool_name must be a nonempty string")
+        return [], TurnToolOutput.model_validate(
+            {"name": input.tool_name, "namespace": input.namespace, "output": input.content}
+        )
+    return _to_wire_input(_normalize_run_input(input)), None