fix(python): verify Lance-created IVF_RQ compatibility (#3793)

## Summary

- add regression coverage for an IVF_RQ index created through the Lance
Python API and consumed through LanceDB
- verify a fresh LanceDB handle can list and query the
externally-created index

## Root cause

Lance 4.0.0-beta.5 could serialize `rotate_mat_position` as null, while
the Lance 2.0.0 reader used by LanceDB 0.29.2 required a `u32`. Upstream
Lance fixed the incompatible IVF_RQ metadata handling and index
versioning, and the current LanceDB dependency includes that repair.
LanceDB was missing coverage for the reported mixed-API workflow,
allowing the compatibility regression to go undetected.

## Validation

- `cd python && uv run --extra tests pytest python/tests/test_index.py
-q` (24 passed)
- `uv run --project python --extra dev ruff format .`
- `uv run --project python --extra dev ruff check .`
- `uv run --project python --extra dev ruff format --check
python/python/tests/test_index.py`
- `uv run --project python --extra dev ruff check
python/python/tests/test_index.py`

Fixes #3092

<!-- lance-gatekeeper-fix:v1 agent=7d8c849bd8421be65d32355c42a60b8a
generation=1 -->

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

python/python/tests/test_index.py

@@ -430,6 +430,28 @@ async def test_create_ivfrq_index(some_table: AsyncTable):
     assert indices[0].name == "vector_idx"
 
 
+@pytest.mark.asyncio
+async def test_read_ivfrq_index_created_with_lance(db_async, some_table: AsyncTable):
+    pytest.importorskip("lance")
+    dataset = await some_table.to_lance()
+    dataset.create_index(
+        "vector",
+        index_type="IVF_RQ",
+        metric="cosine",
+        num_partitions=1,
+    )
+
+    # Open a fresh LanceDB handle so the index is discovered from disk.
+    table = await db_async.open_table("some_table")
+    indices = await table.list_indices()
+    assert len(indices) == 1
+    assert indices[0].index_type == "IvfRq"
+    assert indices[0].columns == ["vector"]
+
+    results = await table.query().nearest_to(list(range(DIM))).limit(8).to_arrow()
+    assert len(results) == 8
+
+
 @pytest.mark.asyncio
 async def test_create_hnswpq_index(some_table: AsyncTable):
     await some_table.create_index("vector", config=HnswPq(num_partitions=10))