Fixes #15497

Problem

The HindsightMemoryProvider can still submit async retain work after provider shutdown has started. During interpreter teardown, asyncio.run_coroutine_threadsafe() fails with RuntimeError: cannot schedule new futures after interpreter shutdown.

Root Cause

shutdown() joins existing threads but has no guard flag to prevent sync_turn() from accepting new work.

Fix

1. Added self._shutting_down = False in __init__() — initializes the guard flag
2. Added early-return guard in sync_turn() — checks self._shutting_down before doing any work
3. Set self._shutting_down = True at top of shutdown() — marks provider as shutting down before joining threads
4. Defense-in-depth in _run_sync() — checks loop.is_closed() before calling asyncio.run_coroutine_threadsafe()

Files Changed

• plugins/memory/hindsight/__init__.py — 7 lines added, 0 removed