## What changed
Make turn event delivery depend on when each consumer attaches:
- Handles returned by `thread.turn(...)` receive events from when the request is sent, including events received before the response arrives.
- Other handles and explicit low-level registrations receive only events from when they join. Remove replay of completed items, token usage, and terminal events; use `thread.read(include_turns=True)` for earlier history.
- Retain unread events for active subscribers and pending starts, release unused turn state, and close the returned subscription when an async start is cancelled.
Preserve the existing low-level subscription when repeated `turn_start(...)` calls return the same turn.
## Testing
Update coverage for late joins, request-time buffering, completion or transport failure before a start returns, async cancellation cleanup, and repeated low-level starts. Adjust external-message integration tests to allow handles to collect different items while receiving the same final response.
GitOrigin-RevId: 3b7f1268df1193dee88385006ae9510101086c24
## 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
