fix(backends): remember the session id from the first frame so a cut turn stays resumable

tests/test_conversations.py

@@ -96,11 +96,13 @@ class ScriptedClient:
     async def receive_response(self):
         prompt = self.prompts[-1]
         yield StreamEvent(
-            uuid="u", session_id="s", event={"type": "message_start", "message": {}}
+            uuid="u",
+            session_id=self.session_id,
+            event={"type": "message_start", "message": {}},
         )
         yield StreamEvent(
             uuid="u",
-            session_id="s",
+            session_id=self.session_id,
             event={
                 "type": "content_block_start",
                 "index": 0,
@@ -109,7 +111,7 @@ class ScriptedClient:
         )
         yield StreamEvent(
             uuid="u",
-            session_id="s",
+            session_id=self.session_id,
             event={
                 "type": "content_block_delta",
                 "index": 0,
@@ -117,7 +119,9 @@ class ScriptedClient:
             },
         )
         yield StreamEvent(
-            uuid="u", session_id="s", event={"type": "content_block_stop", "index": 0}
+            uuid="u",
+            session_id=self.session_id,
+            event={"type": "content_block_stop", "index": 0},
         )
         yield AssistantMessage(content=[TextBlock(text=f"ok:{prompt}")], model="m")
         hold = ScriptedClient.hold
@@ -505,6 +509,7 @@ async def test_graceful_stop_keeps_running_turn_for_recover(world: World) -> Non
     await world.conversations.stop()
     row = await world.conversations.get(conv.external_id)
     assert row.running_turn is not None
+    assert row.session_id == ScriptedClient.instances[0].session_id
     cut = await world.conversations.recover()
     assert [c.external_id for c in cut] == [conv.external_id]
     assert (await world.conversations.get(conv.external_id)).running_turn is None