fix(hindsight): route flush-on-switch through writer queue, not raw thread

Follow-up to the cherry-picked PR #17447. The original flush spawned a
bare threading.Thread for the buffer-flush path, overwriting
self._sync_thread — which is aliased to the long-lived writer thread.
Two consequences:

1. No serialization with the writer queue. If old-session retains were
   still queued in _retain_queue, the flush ran concurrently with the
   writer and both threads could call aretain_batch against the same
   document_id.
2. The pre-spawn 'self._sync_thread.join(timeout=5.0)' tried to join the
   long-lived writer, which never exits, so the join was a no-op that
   just timed out — never actually serialized anything.

Fix: enqueue the flush closure on _retain_queue via _ensure_writer +
put(). Natural FIFO ordering behind any pending retains, no new thread,
no broken join. Shutdown-aware so it doesn't enqueue after teardown.

Tests updated to drain via _retain_queue.join() instead of the stale
_sync_thread.join(). Added regression guard
test_flush_serializes_behind_pending_retains_via_writer_queue that
blocks the writer mid-retain to prove the flush waits in FIFO behind
the old retain.

Also seeds _retain_queue / _shutting_down / stubbed _ensure_writer on
the bare-object test helper in test_memory_session_switch.py so that
path doesn't blow up under the new queue-enqueue.

tests/plugins/memory/test_hindsight_provider.py + tests/agent/test_memory_session_switch.py: 103/103 passing.

tests/plugins/memory/test_hindsight_provider.py

@@ -940,16 +940,17 @@ class TestSessionSwitchBufferFlush:
         p = provider_with_config(retain_every_n_turns=3, retain_async=False)
         old_doc = p._document_id
 
-        # Two turns buffered, no retain yet (boundary is at turn 3).
+        # Two turns buffered, no retain yet (boundary is at turn 3). The
+        # writer hasn't been started either — sync_turn's early return
+        # skips _ensure_writer when no retain is due.
         p.sync_turn("turn1-user", "turn1-asst")
         p.sync_turn("turn2-user", "turn2-asst")
         assert p._sync_thread is None
         p._client.aretain_batch.assert_not_called()
 
-        # Switch — flush should fire under OLD document_id.
+        # Switch — flush should fire under OLD document_id via the writer queue.
         p.on_session_switch("new-sid", parent_session_id="test-session", reset=True)
-        if p._sync_thread:
-            p._sync_thread.join(timeout=5.0)
+        p._retain_queue.join()
 
         p._client.aretain_batch.assert_called_once()
         kw = p._client.aretain_batch.call_args.kwargs
@@ -974,8 +975,8 @@ class TestSessionSwitchBufferFlush:
     def test_no_flush_when_buffer_empty(self, provider):
         """Switch with no buffered turns must not fire a spurious retain."""
         provider.on_session_switch("new-sid")
-        if provider._sync_thread:
-            provider._sync_thread.join(timeout=5.0)
+        # Nothing enqueued — join is immediate.
+        provider._retain_queue.join()
         provider._client.aretain_batch.assert_not_called()
         assert provider._session_id == "new-sid"
 
@@ -1015,6 +1016,61 @@ class TestSessionSwitchBufferFlush:
         assert finished.is_set(), "switch returned before prefetch thread settled"
         assert provider._prefetch_result == ""
 
+    def test_flush_serializes_behind_pending_retains_via_writer_queue(
+        self, provider_with_config
+    ):
+        """The flush closure must ride the same _retain_queue sync_turn
+        uses, so it lands FIFO behind any still-queued old-session
+        retains rather than racing them on a separate thread.
+
+        Regression guard: an earlier draft spawned a raw threading.Thread
+        for flush, overwriting _sync_thread and racing the writer against
+        the same document_id.
+        """
+        import threading as _threading
+
+        p = provider_with_config(retain_every_n_turns=2, retain_async=False)
+
+        # Block the first writer job until we've enqueued the flush
+        # behind it. This proves ordering — the flush MUST wait.
+        gate = _threading.Event()
+        call_order: list[str] = []
+
+        def _aretain_batch_tracking(**kw):
+            idx = kw["items"][0]["metadata"].get("turn_index", "")
+            call_order.append(str(idx))
+            if idx == "2":
+                # First retain blocks until we've enqueued the flush.
+                gate.wait(timeout=5.0)
+
+        p._client.aretain_batch = AsyncMock(side_effect=_aretain_batch_tracking)
+
+        # Turn 1+2 → boundary hit → retain enqueued (will block).
+        p.sync_turn("turn1-user", "turn1-asst")
+        p.sync_turn("turn2-user", "turn2-asst")
+
+        # One more buffered turn so flush has something to land.
+        p.sync_turn("turn3-user", "turn3-asst")
+
+        # Switch while the first retain is still blocked on `gate`.
+        p.on_session_switch("new-sid", parent_session_id="test-session")
+
+        # Release the first retain. Flush must have been enqueued
+        # BEHIND it, and run second.
+        gate.set()
+        p._retain_queue.join()
+
+        # The flush carries all buffered turns; sync_turn's retain #2
+        # carried the batch at boundary time. Two distinct calls.
+        assert p._client.aretain_batch.call_count == 2
+        # First call landed while buffer was [t1, t2]; flush landed
+        # after we added t3. So the second call must be strictly after.
+        assert call_order[0] == "2"
+        # Flush retain has turn_index matching the buffered count at
+        # switch time (3 turns accumulated, _turn_index was set to 3
+        # by the last sync_turn).
+        assert call_order[1] == "3"
+
 
 # ---------------------------------------------------------------------------
 # System prompt tests