style(test): hoist the sleeps collector out of the transient-failure loop

CI / Python quality failed on d5dc9b2: the pylint hook reported

    tests/unit/test_local_embedding_store.py:935:16:
    W0640: Cell variable sleeps defined in loop (cell-var-from-loop)

`fake_sleep` closes over `sleeps` while the loop rebinds it on every
iteration. The collector is now created once before the loop and cleared
at the top of each iteration, which keeps the per-iteration assertion
(`assert sleeps == [1.0]`) exactly as strict and removes the loop-scoped
rebinding.

Verification (repo hook arguments):
  pylint  -> Your code has been rated at 10.00/10, exit 0
  black --check --line-length=120 --target-version=py311 -> clean
  flake8 --extend-ignore=E203 --max-line-length=120       -> clean

tests/unit/test_local_embedding_store.py

@@ -928,11 +928,13 @@ def test_per_item_fallback_keeps_a_transiently_failed_vector(monkeypatch):
             ConnectionError("connection reset"),
             TimeoutError("request timed out"),
         )
-        for transient in transients:
-            sleeps = []
+        sleeps = []
 
-            async def fake_sleep(delay):
-                sleeps.append(delay)
+        async def fake_sleep(delay):
+            sleeps.append(delay)
+
+        for transient in transients:
+            sleeps.clear()
 
             provider = PerItemLimitThenTransientErrorAsEmbedding(transient)
             store = LocalEmbeddingStore(