* test(runtime): add Blockbuster runtime anchor for JsonlRunEventStore async IO
#3084 offloaded `JsonlRunEventStore`'s file IO via `asyncio.to_thread` and added
a mock-based offload assertion (`tests/test_jsonl_event_store_async_io.py`) that
covers `put()` only. That guard is not part of the Blockbuster runtime gate
(`tests/blocking_io/`) run by `backend-blocking-io-tests.yml`.
Add a runtime anchor that drives the full async surface (`put`, `put_batch`,
`list_messages`, `list_events`, `list_messages_by_run`, `count_messages`,
`delete_by_run`, `delete_by_thread`) under the strict Blockbuster gate, so any
blocking IO reintroduced on the event loop in any of these methods fails CI —
not only removal of a specific `to_thread` call. Verified each offloaded method
goes red when its offload is reverted. Test-only; no production change.
* test(runtime): exercise list_events event_types filter branch
Per review feedback: the anchor called list_events without event_types,
so the filter branch never ran after _read_run_events' filesystem IO.
Add a second list_events call with event_types=["message"] so the full
read path -- including the filter branch -- executes under the gate.
