`OpenVikingMemoryProvider.shutdown()` joins in-flight writers, deferred-commit
threads, and prefetch threads, but not `_runtime_start_thread` — the tracked
`daemon=True` waiter that runs `_finish_runtime_openviking_start`, which blocks
on network health probes (`_wait_for_openviking_health` polling + a
`_VikingClient.health()` request).
If the local OpenViking runtime is slow or unreachable, that waiter can still
be blocked in network I/O at interpreter exit. CPython then forcibly kills it
during `Py_FinalizeEx` (`PyThread_exit_thread` -> `__pthread_unwind` ->
`abort()`), producing SIGABRT (exit 134) with no traceback — the same daemon-
thread-at-exit failure class fixed for the Honcho provider.
Fix:
- `shutdown()` now joins `_runtime_start_thread` (timeout-bounded) alongside the
other tracked threads.
- `_wait_for_openviking_health()` gains a `should_stop` callback; the waiter
passes `lambda: self._shutting_down` so the poll loop bails out promptly once
`shutdown()` flips the flag, instead of lingering up to the 60s autostart
timeout and timing out the join (which would leave the thread alive).
- Add tests/plugins/memory/test_openviking_shutdown.py covering the short-circuit
and the shutdown-joins-runtime-thread behaviour.
(cherry picked from commit 5471ec70210b35462450aa52f0cd483439fffba7)
