test(python): cover local sync multiple-vector search (#3830)

## Summary
- add regression coverage for multiple query vectors in the local
synchronous Python API
- verify that each query vector receives its own limited
nearest-neighbor result and `query_index`

## Root cause
In LanceDB v0.16, the local synchronous scanner passed a nested vector
array as one query, unlike the async and remote implementations. The
subsequent sync-to-async table migration supplied the correct shared
runtime path, but this local sync behavior was never regression-tested
and issue #1857 remained open.

## Validation
- `uv run --extra tests pytest
python/tests/test_query.py::test_query_multiple_vectors -q`
- `uv run --project python --extra tests --extra dev ruff format --check
python/python/tests/test_query.py`
- `uv run --project python --extra tests --extra dev ruff check .`

Fixes #1857

<!-- lance-gatekeeper-fix:v1 agent=6b25bc529d76813c3db7627c8be947ef
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/tests/test_query.py

@@ -570,6 +570,15 @@ def test_query_builder(table):
     assert all(np.array(rs[0]["vector"]) == [1, 2])
 
 
+def test_query_multiple_vectors(table):
+    results = table.search([np.array([1, 2]), np.array([4, 5])]).limit(1).to_list()
+
+    assert len(results) == 2
+    results_by_query = {result["query_index"]: result for result in results}
+    assert results_by_query[0]["id"] == 1
+    assert results_by_query[1]["id"] == 2
+
+
 def test_with_row_id(table: lancedb.table.Table):
     rs = table.search().with_row_id(True).to_arrow()
     assert "_rowid" in rs.column_names