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_remote_db.py

@@ -17,7 +17,7 @@ from packaging.version import Version
 
 import lancedb
 from lancedb.conftest import MockTextEmbeddingFunction
-from lancedb.query import ColumnOrdering
+from lancedb.query import AsyncQuery, ColumnOrdering
 from lancedb.remote import ClientConfig
 from lancedb.remote.errors import HttpError, RetryError
 import pytest
@@ -1521,6 +1521,46 @@ def test_query_sync_empty_query():
         assert data == expected
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("search_kwargs", [{}, {"query": None}])
+async def test_async_search_without_query(search_kwargs):
+    expected = pa.table({"id": [7, 8, 9]})
+
+    def handler(request):
+        if request.path == "/v1/table/test/describe/":
+            request.send_response(200)
+            request.send_header("Content-Type", "application/json")
+            request.end_headers()
+            request.wfile.write(b'{"version": 1, "schema": {"fields": []}}')
+        elif request.path == "/v1/table/test/query/":
+            body = json.loads(
+                request.rfile.read(int(request.headers["Content-Length"]))
+            )
+            assert body == {
+                "k": 3,
+                "filter": "id >= 7",
+                "vector": [],
+                "columns": ["id"],
+                "prefilter": True,
+                "version": None,
+            }
+            request.send_response(200)
+            request.send_header("Content-Type", "application/vnd.apache.arrow.file")
+            request.end_headers()
+            with pa.ipc.new_file(request.wfile, schema=expected.schema) as writer:
+                writer.write_table(expected)
+        else:
+            request.send_response(404)
+            request.end_headers()
+
+    async with mock_lancedb_connection_async(handler) as db:
+        table = await db.open_table("test")
+        query = await table.search(**search_kwargs)
+        assert isinstance(query, AsyncQuery)
+        result = await query.where("id >= 7").select(["id"]).limit(3).to_arrow()
+        assert result == expected
+
+
 def test_query_sync_maximal():
     def handler(body):
         assert body == {