feat(python): expose fast_search in synchronous API (Fixes #2612) (#2962)

Fixes #2612

This PR exposes the private _fast_search attribute via a public
fast_search() method in the synchronous LanceVectorQueryBuilder.

Previously, enabling fast search in the sync API required accessing a
private member (query._fast_search = True). This change aligns the
synchronous API with the Async and Remote APIs, allowing for cleaner,
more Pythonic method chaining.

Changes:
Added fast_search() method to LanceVectorQueryBuilder in
python/python/lancedb/query.py.
Added a unit test verifying the flag works with high-dimensional data
(2560 dims) and chaining.
Example Usage:

Before:

```
query = table.search(vector)
query._fast_search = True  # Private attribute usage
results = query.limit(10).to_pandas()
```

After:

```
results = (
    table.search(vector)
    .fast_search()
    .limit(10)
    .to_pandas()
)
```

Verification:
I have added a test case (test_fast_search_high_dimension) that
replicates the scenario described in the issue (2560 dimensions, cosine
distance) to ensure the pipeline constructs the query correctly without
errors.

Checklist:

- [ ]  I have added tests to cover my changes.
- [ ]  All new and existing tests passed.
- [ ]  Documentation has been updated (inline docstrings).

Signed-off-by: Rashidul Islam <rasidulislam71@gmail.com>

python/python/lancedb/query.py

@@ -1428,6 +1428,19 @@ class LanceVectorQueryBuilder(LanceQueryBuilder):
         self._bypass_vector_index = True
         return self
 
+    def fast_search(self) -> LanceVectorQueryBuilder:
+        """
+        Skip a flat search of unindexed data. This will improve
+        search performance but search results will not include unindexed data.
+
+        Returns
+        -------
+        LanceVectorQueryBuilder
+            The LanceVectorQueryBuilder object.
+        """
+        self._fast_search = True
+        return self
+
 
 class LanceFtsQueryBuilder(LanceQueryBuilder):
     """A builder for full text search for LanceDB."""

python/python/tests/test_query.py

@@ -1499,3 +1499,30 @@ def test_search_empty_table(mem_db):
     # Search on empty table should return empty results, not crash
     results = table.search([1.0, 2.0]).limit(5).to_list()
     assert results == []
+
+
+def test_fast_search(tmp_path):
+    db = lancedb.connect(tmp_path)
+
+    # Generate data matching the async test style
+    vectors = pa.FixedShapeTensorArray.from_numpy_ndarray(
+        np.random.rand(256, 32)
+    ).storage
+
+    table = db.create_table("test", pa.table({"vector": vectors}))
+
+    # FIX: Pass arguments directly instead of using 'config=IvfPq(...)'
+    table.create_index(vector_column_name="vector", num_partitions=1, num_sub_vectors=1)
+
+    # Add data to ensure table has enough segments/rows
+    table.add(pa.table({"vector": vectors}))
+
+    q = [1.0] * 32
+
+    # 1. Normal Search -> Should include "LanceScan" (Brute Force / Scan)
+    plan = table.search(q).explain_plan(True)
+    assert "LanceScan" in plan
+
+    # 2. Fast Search -> Should NOT include "LanceScan" (Uses Index)
+    plan = table.search(q).fast_search().explain_plan(True)
+    assert "LanceScan" not in plan