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/docs/api-reference.md

@@ -31,6 +31,7 @@ from openai_codex import (
     LocalImageInput,
     SkillInput,
     MentionInput,
+    ExternalMessage,
 )
 from openai_codex.types import (
     Account,
@@ -201,7 +202,7 @@ These options have the same behavior on sync and async `run(...)` and `turn(...)
 | `turn_service_tier: str | None = None` | Overrides the tier for a newly started turn only. `None` inherits the thread setting; `"default"` selects standard speed. Does not change the thread default and is ignored when input joins an active turn. |
 | `source: str | None = None` | Labels the caller that initiated a new turn, such as `"review_ui"`. This is metadata; it does not schedule work or grant authority. Ignored when input joins an active turn. |
 
-`turn_service_tier`, `source`, and explicit `include_turns`
+`ExternalMessage`, `turn_service_tier`, `source`, and explicit `include_turns`
 on resume/fork require Codex CLI 0.151.0 or newer. The SDK raises `CodexError`
 before sending these options to an older runtime, which would otherwise ignore
 them. Published SDK releases install a matching runtime automatically; when
@@ -267,7 +268,7 @@ Behavior notes:
 
 InputItem = TextInput | ImageInput | LocalImageInput | SkillInput | MentionInput
 Input = list[InputItem] | InputItem
-RunInput = Input | str
+RunInput = Input | str | ExternalMessage
 ```
 
 Use `ImageInput` with a base64-encoded `data:image/...` URL. HTTP and HTTPS image URLs are
@@ -276,6 +277,58 @@ deprecated; download remote images and pass their local paths with `LocalImageIn
 Use a plain `str` as shorthand for `TextInput(...)` anywhere a turn input is accepted:
 `thread.run("...")`, `thread.turn("...")`, and `turn.steer("...")`.
 
+### ExternalMessage
+
+`ExternalMessage` supplies **untrusted content** from another agent, tool, or
+application. Content reaches the model with tool-level authority, below user
+and developer instructions. It does not establish user authorization or
+approval. Keep the thread's sandbox and approval policies appropriate for the
+work the user has authorized.
+
+```python
+from openai_codex import ExternalMessage
+
+message = ExternalMessage(
+    tool_name="notifications",
+    namespace="slack",
+    content="Deployment notification: the staging checks failed.",
+)
+result = thread.run(message)
+```
+
+| Field | Meaning |
+| --- | --- |
+| `tool_name: str` | Required, nonempty name of the tool or application delivering the message. |
+| `content` | Required text, or a sequence of structured content dictionaries or generated `FunctionCallOutputContentItem` models. Structured image content requires inline data URLs. |
+| `namespace: str | None = None` | Optional namespace for the tool name. |
+
+Pass one `ExternalMessage` as the complete input to `run(...)` or `turn(...)`.
+It starts a turn when the thread is idle or joins an active regular turn. It
+appears in saved history and item notifications as a `functionCallOutput`
+item, retaining tool authority. No preceding tool call or call ID is required.
+Tool names and namespaces identify the source; they are not proof of its
+identity or permission to act.
+
+When a message joins an active turn, both handles can stream or collect the
+result independently. A joining handle receives previously completed items and
+the latest usage, followed by live notifications. Consumed transient events such
+as token deltas are discarded. Both handles collect the complete result, and
+closing one stream leaves the other active.
+
+The async calls use the same object:
+
+```python
+result = await async_thread.run(message)
+```
+
+Use `await async_thread.turn(message)` to collect a handle for streaming and
+interruption. An `ExternalMessage` cannot be mixed into a user-input list.
+`TurnHandle.steer(...)` accepts user input; deliver an external message to an
+active turn through `thread.turn(message)`.
+
+See the [external message examples](../examples/16_external_message) for a user
+request followed by an external notification.
+
 ## Public Types
 
 The SDK wrappers return and accept public Codex protocol models wherever possible: