feat(core): finalize session event lifecycle (#35272)

packages/llm/test/provider/openai-responses.test.ts

@@ -764,6 +764,35 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
+  // OpenAI's documented stream orders output text within one message item; no
+  // provider-valid same-kind overlap is evidenced, so done boundaries close it.
+  it.effect("closes sequential output messages before starting the next", () =>
+    Effect.gen(function* () {
+      const response = yield* LLMClient.generate(request).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_text.delta", item_id: "msg_1", delta: "First" },
+              { type: "response.output_text.done", item_id: "msg_1" },
+              { type: "response.output_text.delta", item_id: "msg_2", delta: "Second" },
+              { type: "response.output_item.done", item: { type: "message", id: "msg_2" } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.events.filter((event) => event.type.startsWith("text-"))).toEqual([
+        { type: "text-start", id: "msg_1" },
+        { type: "text-delta", id: "msg_1", text: "First" },
+        { type: "text-end", id: "msg_1" },
+        { type: "text-start", id: "msg_2" },
+        { type: "text-delta", id: "msg_2", text: "Second" },
+        { type: "text-end", id: "msg_2" },
+      ])
+    }),
+  )
+
   it.effect("parses reasoning summary stream fixtures", () =>
     Effect.gen(function* () {
       const body = sseEvents(