Change `for` to `async for` in VertexAiSessionService.list_sessions() so
that the AsyncPager returned by the API client fetches subsequent pages.
Improve the test mocks so they only support async iteration (via the
existing to_async_iterator helper) instead of AsyncIterableList(list) which
silently allows synchronous `for` and would not catch a regression.
Add a dedicated pagination regression test
(test_list_sessions_pagination_keeps_client_open) that verifies iteration
happens inside the client context across multiple pages, mirroring the
existing get_session pagination test.
Rename the misleading test_list_sessions_with_pagination to
test_list_sessions_filtered_by_user since it tests user filtering, not
pagination.
https://claude.ai/code/session_01KRLYDR2mPepKZjW6rm7Xt8
