fix: Preserve rrf_ranking_alpha=0.0 in MatchingEngineIndexEndpoint hybrid queries

`HybridQuery.rrf_ranking_alpha` is an optional float that defaults to
`None`. Two call sites in `matching_engine_index_endpoint.py` previously
gated RRF wiring on the truthiness of this field
(`if query.rrf_ranking_alpha`), which incorrectly treated an explicit
`0.0` the same as an unset value. As a result, callers passing
`rrf_ranking_alpha=0.0` (pure sparse ranking) had the `rrf` submessage
silently omitted from the outgoing request, and the server fell back to
its default alpha instead of honoring the caller's intent.

Switch both sites to `is not None` so `0.0` is preserved end-to-end:
- `find_neighbors` (public path, `FindNeighborsRequest.Query.RRF`)
- `match` (private-service-access path, `MatchRequest.RRF`)

Extend `_TEST_HYBRID_QUERIES` with a sparse-only entry that sets
`rrf_ranking_alpha=0.0`, and update the two consumers
(`test_private_service_access_hybrid_search_match_queries` and
`test_index_public_endpoint_find_neighbors_queries`) to assert that the
resulting RPC contains `rrf { alpha: 0.0 }` (instead of no rrf at all).
Also switch the expected-side predicate in the private-service-access
test from truthiness to `is not None` for consistency with the SDK.

FUTURE_COPYBARA_INTEGRATE_REVIEW=#6967 from googleapis:release-please--branches--main 4a0d1d2
PiperOrigin-RevId: 948540018

Author: vertex-sdk-bot

google/cloud/aiplatform/matching_engine/matching_engine_index_endpoint.py

@@ -1865,7 +1865,7 @@ def find_neighbors(
                         dimensions=query.sparse_embedding_dimensions,
                     ),
                 )
-                if query.rrf_ranking_alpha:
+                if query.rrf_ranking_alpha is not None:
                     find_neighbors_query.rrf = (
                         gca_match_service_v1beta1.FindNeighborsRequest.Query.RRF(
                             alpha=query.rrf_ranking_alpha,
@@ -2200,7 +2200,7 @@ def match(
                         match_service_pb2.MatchRequest.RRF(
                             alpha=query.rrf_ranking_alpha,
                         )
-                        if query_is_hybrid and query.rrf_ranking_alpha
+                        if query_is_hybrid and query.rrf_ranking_alpha is not None
                         else None
                     ),
                 )

tests/unit/aiplatform/test_matching_engine_index_endpoint.py

@@ -258,6 +258,11 @@
         sparse_embedding_dimensions=[1, 2, 3],
         sparse_embedding_values=[0.1, 0.2, 0.3],
     ),
+    HybridQuery(
+        sparse_embedding_dimensions=[1, 2, 3],
+        sparse_embedding_values=[0.1, 0.2, 0.3],
+        rrf_ranking_alpha=0.0,  # Test edge case where alpha is 0
+    ),
 ]
 _TEST_NUM_NEIGHBOURS = 1
 _TEST_FILTER = [
@@ -1539,7 +1544,7 @@ def test_private_service_access_hybrid_search_match_queries(
                                 match_service_pb2.MatchRequest.RRF(
                                     alpha=_TEST_HYBRID_QUERIES[i].rrf_ranking_alpha,
                                 )
-                                if _TEST_HYBRID_QUERIES[i].rrf_ranking_alpha
+                                if _TEST_HYBRID_QUERIES[i].rrf_ranking_alpha is not None
                                 else None
                             ),
                         )
@@ -2093,6 +2098,27 @@ def test_index_public_endpoint_find_neighbors_queries(
                     approximate_neighbor_count=_TEST_APPROX_NUM_NEIGHBORS,
                     fraction_leaf_nodes_to_search_override=_TEST_FRACTION_LEAF_NODES_TO_SEARCH_OVERRIDE,
                 ),
+                gca_match_service_v1beta1.FindNeighborsRequest.Query(
+                    neighbor_count=_TEST_NUM_NEIGHBOURS,
+                    datapoint=gca_index_v1beta1.IndexDatapoint(
+                        restricts=[
+                            gca_index_v1beta1.IndexDatapoint.Restriction(
+                                namespace="class",
+                                allow_list=["token_1"],
+                                deny_list=["token_2"],
+                            )
+                        ],
+                        sparse_embedding=gca_index_v1beta1.IndexDatapoint.SparseEmbedding(
+                            values=[0.1, 0.2, 0.3], dimensions=[1, 2, 3]
+                        ),
+                    ),
+                    rrf=gca_match_service_v1beta1.FindNeighborsRequest.Query.RRF(
+                        alpha=0.0,
+                    ),
+                    per_crowding_attribute_neighbor_count=_TEST_PER_CROWDING_ATTRIBUTE_NUM_NEIGHBOURS,
+                    approximate_neighbor_count=_TEST_APPROX_NUM_NEIGHBORS,
+                    fraction_leaf_nodes_to_search_override=_TEST_FRACTION_LEAF_NODES_TO_SEARCH_OVERRIDE,
+                ),
             ],
             return_full_datapoint=_TEST_RETURN_FULL_DATAPOINT,
         )