fix(vector-store): protect bulk insert mode transitions with lock

Ensure begin_bulk_insert() and end_bulk_insert() are fully
lock-protected to prevent TOCTOU race conditions.

Solution-ID: SOL-1735392000003
Issue-ID: ISS-1766921318981-12
Task-ID: T2

codex-lens/tests/test_vector_store.py

@@ -1,5 +1,6 @@
 import tempfile
 import threading
+import time
 from pathlib import Path
 
 import numpy as np
@@ -65,3 +66,66 @@ def test_concurrent_bulk_insert(monkeypatch: pytest.MonkeyPatch, temp_db: Path)
     assert len(store._bulk_insert_embeddings) == 50
     assert store.count_chunks() == 50
 
+
+def test_bulk_insert_mode_transitions(monkeypatch: pytest.MonkeyPatch, temp_db: Path) -> None:
+    """begin/end bulk insert should be thread-safe with concurrent add operations."""
+    store = VectorStore(temp_db)
+
+    class DummyAnn:
+        def __init__(self) -> None:
+            self.total_added = 0
+            self.save_calls = 0
+
+        def add_vectors(self, ids, embeddings) -> None:
+            self.total_added += len(ids)
+
+        def save(self) -> None:
+            self.save_calls += 1
+
+    dummy_ann = DummyAnn()
+    store._ann_index = dummy_ann
+    monkeypatch.setattr(store, "_ensure_ann_index", lambda dim: True)
+
+    errors: list[Exception] = []
+    lock = threading.Lock()
+    stop_event = threading.Event()
+
+    def adder(worker_id: int) -> None:
+        try:
+            while not stop_event.is_set():
+                chunk = SemanticChunk(content=f"chunk {worker_id}", metadata={})
+                chunk.embedding = np.random.randn(8).astype(np.float32)
+                store.add_chunks_batch([(chunk, f"file_{worker_id}.py")], auto_save_ann=False)
+        except Exception as exc:
+            with lock:
+                errors.append(exc)
+
+    def toggler() -> None:
+        try:
+            for _ in range(5):
+                store.begin_bulk_insert()
+                time.sleep(0.05)
+                store.end_bulk_insert()
+                time.sleep(0.05)
+        except Exception as exc:
+            with lock:
+                errors.append(exc)
+
+    threads = [threading.Thread(target=adder, args=(i,)) for i in range(3)]
+    toggle_thread = threading.Thread(target=toggler)
+
+    for t in threads:
+        t.start()
+    toggle_thread.start()
+
+    toggle_thread.join(timeout=10)
+    stop_event.set()
+    for t in threads:
+        t.join(timeout=10)
+
+    assert not errors
+    assert toggle_thread.is_alive() is False
+    assert store._bulk_insert_mode is False
+    assert store._bulk_insert_ids == []
+    assert store._bulk_insert_embeddings == []
+    assert dummy_ann.total_added == store.count_chunks()