docs(chat): warn about the two upgrade hazards in the changesets

Both stay patch. The double write only bites code that worked around a lost
message, and the silent action completion was the bug it now reports, so
neither is new functionality or an API break. The version cannot carry either
signal, so the changelog entries name them instead.

Also documents sendPendingMessage in the testing harness table, which listed
every other send method.

Author: ericallam

.changeset/action-stream-into-conversation.md

@@ -4,4 +4,4 @@
 
 A response streamed back from `onAction` is now part of the conversation. Returning a `StreamTextResult` from an action sent it to the browser and nowhere else, so a regenerate showed the user a new answer that the model had no memory of, and the next turn carried on from the answer it had replaced.
 
-A stream that fails part-way through is also no longer committed as though it finished. Whatever streamed is still kept, but the failure is reported instead of the truncated text being stored, and built on, as a complete answer.
+A stream that fails part-way through is also no longer committed as though it finished. Whatever streamed is still kept, but the failure is reported instead of the truncated text being stored, and built on, as a complete answer. An action that used to end quietly on a mid-stream failure now surfaces an error to the frontend, so handle it the way you handle a failed turn.

.changeset/steering-messages-accumulator.md

@@ -3,3 +3,5 @@
 ---
 
 Steering messages injected mid-answer are now part of the conversation your hooks see. Previously they reached the model and the browser but not `onTurnComplete`, so an app storing its own transcript lost the instruction the answer was shaped by. It vanished from the conversation on reload, and later turns had no record of it.
+
+If you worked around this by saving steering messages as they arrive, in `pendingMessages.onReceived` for example, that write now duplicates the one you get from `newUIMessages`. Drop it, or skip messages you have already stored.

docs/ai-chat/testing.mdx

@@ -634,6 +634,7 @@ The harness's initial wire payload depends on `mode`:
 | `sendHandover({ partialAssistantMessage, isFinal?, messageId? })` | Dispatch a `handover` signal — only meaningful when started with `mode: "handover-prepare"`. The agent picks up partial assistant messages and continues the turn. |
 | `sendHandoverSkip()`                  | Dispatch a `handover-skip` signal — only meaningful when started with `mode: "handover-prepare"`. The agent exits cleanly without firing turn hooks. |
 | `sendAction(action)`                  | Route a custom action through `actionSchema` + `onAction`.                                             |
+| `sendPendingMessage(message)`         | Append a user message mid-turn without waiting for a turn to complete, so it reaches the running turn as a steering message. Resolves once the record has landed on `session.in`. |
 | `sendStop(message?)`                  | Fire a stop signal. Does not wait for the turn — the run's `signal.aborted` becomes `true`.            |
 | `seedSnapshot(snapshot)`              | Pre-seed the snapshot read for the next boot. Effective on the next run boot only.                     |
 | `seedSessionOutTail(chunks?)`         | Pre-seed `session.out` chunks for the next boot's replay. Reduces to settled assistant turns.          |