test: fix vector progress callback timing assertion (#6437)

This updates the vector progress-callback test to stop treating
`complete:merge_partitions` as if the full `create_index` operation has
already finished. That assumption is timing-dependent because the final
dataset commit happens after partition merge completes, which made the
test fail on Windows while the implementation was still behaving as
designed.

The revised test now checks a deterministic earlier failure point: if
the callback raises at `complete:shuffle`, `create_index` should
propagate the error and leave the dataset without a committed index.
Validation was limited to a local `python3 -m py_compile
python/python/tests/test_vector_index.py` syntax check; I did not run
the full Python test suite locally because building the editable
extension would require a longer Rust/Python release build.

Author: Xuanwo

python/python/tests/test_vector_index.py

@@ -219,24 +219,25 @@ def test_create_index_progress_callback_vector(tmp_path):
     assert merge_progress[-1] == 4
 
 
-def test_create_index_progress_callback_error_after_completion_is_ignored(tmp_path):
+def test_create_index_progress_callback_error_before_completion_propagates(tmp_path):
     ds = _make_sample_dataset_base(
         tmp_path, "vector_progress_post_commit_error", 1500, 128
     )
-    recorder = ProgressRecorder(fail_on_tag="complete:merge_partitions")
+    recorder = ProgressRecorder(fail_on_tag="start:train_ivf")
 
-    ds.create_index(
-        column="vector",
-        index_type="IVF_PQ",
-        num_partitions=4,
-        num_sub_vectors=4,
-        progress_callback=recorder,
-    )
+    with pytest.raises(RuntimeError, match="progress callback failure"):
+        ds.create_index(
+            column="vector",
+            index_type="IVF_PQ",
+            num_partitions=4,
+            num_sub_vectors=4,
+            progress_callback=recorder,
+        )
 
     tags = progress_event_tags(recorder.events)
-    assert tags[-1] == "complete:merge_partitions"
-    assert ds.has_index
-    assert ds.describe_indices()[0].field_names == ["vector"]
+    assert tags == ["start:train_ivf"]
+    assert not ds.has_index
+    assert ds.describe_indices() == []
 
 
 def test_distributed_ivf_pq_partition_window_env_override(tmp_path, monkeypatch):