fix(sse): coerce non-int ui._event_id to None when stamping saves

When the active UI is a MagicMock test double, _ui_event_id() returned
the auto-vivified _event_id mock (getattr finds it, so the None default
never applies). That mock reached the conversations INSERT and failed
to bind ("type 'MagicMock' is not supported"), so save_message raised,
the row was dropped, and tests on the real-storage + mock-UI path broke
(CI: test_session_attachments::test_db_row_stores_text_only).

Coerce a non-int _event_id to None so mock UIs -- and counterless
CLI/eval/placeholder UIs -- stamp NULL (the synthetic-snapshot floor),
matching the documented contract. Production UIs always carry an int,
so behaviour there is unchanged.

Also drop two redundant local `import json` in the new /history
integration tests; the module-level import already covers them.

tests/test_workstream_endpoints.py

@@ -1304,8 +1304,6 @@ class TestHistoryInteractive:
         ``cursor`` = the resolved boundary's event_id.  The client opens
         its initial SSE with that cursor so the delta rebuilds the turn.
         """
-        import json
-
         ws_id = "ws-cursor"
         _inject_storage.register_workstream(ws_id, kind="interactive", user_id="test-user")
         _inject_storage.save_message(ws_id, "user", "kick off", event_id=10)
@@ -1340,8 +1338,6 @@ class TestHistoryInteractive:
         rendered block) and returns ``cursor: null`` — the client connects
         fresh to the synthetic-snapshot floor, never leaving the turn
         unrenderable."""
-        import json
-
         ws_id = "ws-cursor-reload"
         _inject_storage.register_workstream(ws_id, kind="interactive", user_id="test-user")
         _inject_storage.save_message(ws_id, "user", "kick off", event_id=10)

turnstone/core/session.py

@@ -2134,12 +2134,15 @@ class ChatSession:
         """Current per-ws SSE ring-buffer high-water mark for stamping
         saved messages with the ``Last-Event-ID`` resume cursor.
 
-        Returns ``getattr(self.ui, "_event_id", None)`` — ``None`` for
-        UIs without the counter (CLI / eval / placeholder), whose rows
-        then stay NULL and are treated by ``/history`` as "no
-        fast-forward cursor available" (the synthetic-snapshot floor).
+        ``None`` for UIs without an integer counter — CLI / eval /
+        placeholder UIs (no ``_event_id``), and test doubles whose
+        ``self.ui`` is a ``MagicMock`` (a non-int ``_event_id`` would
+        otherwise reach the INSERT and fail to bind).  Those rows stay
+        NULL and are treated by ``/history`` as "no fast-forward cursor
+        available" (the synthetic-snapshot floor).
         """
-        return getattr(self.ui, "_event_id", None)
+        eid = getattr(self.ui, "_event_id", None)
+        return eid if isinstance(eid, int) else None
 
     def _remaining_token_budget(self) -> int:
         """Estimate how many tokens are available for new content.