fix(python): support async search without a query (#4419)

`AsyncTable.search()` and `search(None)` reached vector search without
assigning `vector_query`, raising `UnboundLocalError` on both local and
remote tables. Return a plain `AsyncQuery` before inferring the search
type, equivalent to `table.query()`, and update the overloads and
documentation to describe that return value.

Add six regression cases for omitted and explicit `None` queries on
local tables with and without vectors and on a mocked remote endpoint.
They verify scans, limits, filters, column projection, and the remote
request body.

Validation: all six regressions reproduced the original
`UnboundLocalError`; all 11 focused query tests pass with the fix,
including existing vector, full-text, hybrid, and sync scan coverage.
The uv development bootstrap, locked Ruff formatting/lint checks, and
focused Pyright return-type assertions also pass.

Fixes #4385

<!-- lance-gatekeeper-fix:v1 agent=bb9a7509f6dadde030d8113b172abc9a
generation=1 -->

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

python/python/tests/test_query.py

@@ -24,6 +24,7 @@ from lancedb.pydantic import LanceModel, Vector
 from lancedb.query import (
     AsyncFTSQuery,
     AsyncHybridQuery,
+    AsyncQuery,
     AsyncQueryBase,
     AsyncVectorQuery,
     ColumnOrdering,
@@ -1279,6 +1280,27 @@ async def test_query_to_polars_async(table_async: AsyncTable):
     assert df.shape == (0, num_columns)
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("search_kwargs", [{}, {"query": None}])
+@pytest.mark.parametrize("with_vector", [False, True])
+async def test_async_search_without_query(mem_db_async, search_kwargs, with_vector):
+    data = [{"id": i} for i in range(10)]
+    if with_vector:
+        for row in data:
+            row["vector"] = [float(row["id"]), 1.0]
+    table = await mem_db_async.create_table("test", data)
+
+    query = await table.search(**search_kwargs)
+    assert isinstance(query, AsyncQuery)
+    assert await query.limit(3).to_arrow() == await table.query().limit(3).to_arrow()
+
+    query = await table.search(**search_kwargs)
+    assert await query.where("id >= 8").select(["id"]).limit(3).to_list() == [
+        {"id": 8},
+        {"id": 9},
+    ]
+
+
 @pytest.mark.asyncio
 async def test_none_query(table_async: AsyncTable):
     with pytest.raises(ValueError):