fix(sdk): include the recovered partial in the error event's newMessages

The error-path onTurnComplete added the partial to newUIMessages/uiMessages but
left newMessages (the model-message delta) empty, so apps persisting the model
delta missed the partial. Populate it from the same conversion.

Author: matt-aitken

packages/trigger-sdk/src/v3/ai.ts

@@ -7960,6 +7960,8 @@ function chatAgent<
               erroredNewUIMessages.push(partialResponse);
             }
 
+            let erroredNewModelMessages: ModelMessage[] = [];
+
             // Commit the complete error state to the canonical accumulator so the
             // errored user message and any recovered partial survive past this
             // hook: the run stays alive after an error, so the next turn sees
@@ -7978,10 +7980,13 @@ function chatAgent<
               accumulatedUIMessages = erroredUIMessagesWithPartial;
               locals.set(chatCurrentUIMessagesKey, accumulatedUIMessages);
               try {
+                if (partialResponse) {
+                  erroredNewModelMessages = await toModelMessages([
+                    stripProviderMetadata(partialResponse),
+                  ]);
+                }
                 if (onlyAppendedPartial) {
-                  accumulatedMessages.push(
-                    ...(await toModelMessages([stripProviderMetadata(partialResponse!)]))
-                  );
+                  accumulatedMessages.push(...erroredNewModelMessages);
                 } else {
                   accumulatedMessages = await toModelMessages(accumulatedUIMessages);
                 }
@@ -8004,7 +8009,7 @@ function chatAgent<
                       chatId: currentWirePayload.chatId,
                       messages: accumulatedMessages,
                       uiMessages: erroredUIMessagesWithPartial,
-                      newMessages: [],
+                      newMessages: erroredNewModelMessages,
                       newUIMessages: erroredNewUIMessages,
                       responseMessage: partialResponse,
                       rawResponseMessage: partialResponse,

packages/trigger-sdk/test/chat-agent-source-stream-error.test.ts

@@ -95,6 +95,19 @@ describe("chat.agent managed loop — source-stream failure", () => {
       // dropped (responseMessage: undefined).
       expect(evt.responseMessage).toBeDefined();
       expect(extractText(evt.responseMessage)).toBe("partial answer");
+
+      const newAssistantText = (evt.newMessages as ModelMessage[])
+        .filter((m) => m.role === "assistant")
+        .map((m) =>
+          typeof m.content === "string"
+            ? m.content
+            : (m.content as Array<{ type: string; text?: string }>)
+                .filter((p) => p.type === "text")
+                .map((p) => p.text ?? "")
+                .join("")
+        )
+        .join("");
+      expect(newAssistantText).toBe("partial answer");
     } finally {
       await harness.close();
     }