tests: resolve failure related to unpacking dictionary for protobuf messages (#17273)

This PR resolves the following failure in
#17245 ([build
log](https://github.com/googleapis/google-cloud-python/actions/runs/26527262176/job/78134171625?pr=17245))

```
==================================== ERRORS ====================================
_ ERROR collecting tests/unit/gapic/bigtable_admin_v2/test_bigtable_instance_admin.py _
tests/unit/gapic/bigtable_admin_v2/test_bigtable_instance_admin.py:7259: in <module>
    iam_policy_pb2.GetIamPolicyRequest({}),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   TypeError: _AddInitMethod.<locals>.init() takes 1 positional argument but 2 were given
_ ERROR collecting tests/unit/gapic/bigtable_admin_v2/test_bigtable_table_admin.py _
tests/unit/gapic/bigtable_admin_v2/test_bigtable_table_admin.py:11222: in <module>
    iam_policy_pb2.GetIamPolicyRequest({}),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   TypeError: _AddInitMethod.<locals>.init() takes 1 positional argument but 2 were given
```


To resolve the issue, this PR adds `**` to unpack fields as keyword
arguments, maintaining compatibility with both proto-plus and pure
protobuf messages.

Author: parthea

packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_macros.j2

@@ -4,7 +4,9 @@
 {%- set auto_populated_field_sample_value = "explicit value for autopopulate-able field" -%}
 {% with method_name = method.client_method_name|snake_case + "_unary" if method.extended_lro and not full_extended_lro else method.client_method_name|snake_case, method_output = method.extended_lro.operation_type if method.extended_lro and not full_extended_lro else method.output %}
 @pytest.mark.parametrize("request_type", [
-  {{ method.input.ident }}({
+{% if auto_populated_fields %}
+  # Pure protobuf messages (non-proto-plus) require keyword arguments.
+  {{ method.input.ident }}(**{
     {% for auto_populated_field in auto_populated_fields %}
     "{{ auto_populated_field }}": "{{ auto_populated_field_sample_value }}",
     {% endfor %}
@@ -14,6 +16,10 @@
     "{{ auto_populated_field }}": "{{ auto_populated_field_sample_value }}",
     {% endfor %}
   },
+{% else %}
+  {{ method.input.ident }}(),
+  {},
+{% endif %}
 ])
 def test_{{ method_name }}(request_type, transport: str = 'grpc'):
     client = {{ service.client_name }}(
@@ -260,7 +266,9 @@ async def test_{{ method_name }}_async_use_cached_wrapped_rpc(transport: str = "
 
 @pytest.mark.asyncio
 @pytest.mark.parametrize("request_type", [
-  {{ method.input.ident }}({
+{% if auto_populated_fields %}
+  # Pure protobuf messages (non-proto-plus) require keyword arguments.
+  {{ method.input.ident }}(**{
     {%- for auto_populated_field in auto_populated_fields %}
     "{{ auto_populated_field }}": "{{ auto_populated_field_sample_value }}",
     {%- endfor %}
@@ -270,6 +278,10 @@ async def test_{{ method_name }}_async_use_cached_wrapped_rpc(transport: str = "
     "{{ auto_populated_field }}": "{{ auto_populated_field_sample_value }}",
     {%- endfor %}
   },
+{% else %}
+  {{ method.input.ident }}(),
+  {},
+{% endif %}
 ])
 async def test_{{ method_name }}_async(request_type, transport: str = 'grpc_asyncio'):
     client = {{ service.async_client_name }}(

packages/gapic-generator/tests/fragments/google/cloud/location/locations.proto

@@ -0,0 +1,108 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+package google.cloud.location;
+
+import "google/api/annotations.proto";
+import "google/protobuf/any.proto";
+import "google/api/client.proto";
+
+option cc_enable_arenas = true;
+option go_package = "google.golang.org/genproto/googleapis/cloud/location;location";
+option java_multiple_files = true;
+option java_outer_classname = "LocationsProto";
+option java_package = "com.google.cloud.location";
+
+// An abstract interface that provides location-related information for
+// a service. Service-specific metadata is provided through the
+// [Location.metadata][google.cloud.location.Location.metadata] field.
+service Locations {
+  option (google.api.default_host) = "cloud.googleapis.com";
+  option (google.api.oauth_scopes) = "https://www.googleapis.com/auth/cloud-platform";
+
+  // Lists information about the supported locations for this service.
+  rpc ListLocations(ListLocationsRequest) returns (ListLocationsResponse) {
+    option (google.api.http) = {
+      get: "/v1/{name=locations}"
+      additional_bindings {
+        get: "/v1/{name=projects/*}/locations"
+      }
+    };
+  }
+
+  // Gets information about a location.
+  rpc GetLocation(GetLocationRequest) returns (Location) {
+    option (google.api.http) = {
+      get: "/v1/{name=locations/*}"
+      additional_bindings {
+        get: "/v1/{name=projects/*/locations/*}"
+      }
+    };
+  }
+}
+
+// The request message for [Locations.ListLocations][google.cloud.location.Locations.ListLocations].
+message ListLocationsRequest {
+  // The resource that owns the locations collection, if applicable.
+  string name = 1;
+
+  // The standard list filter.
+  string filter = 2;
+
+  // The standard list page size.
+  int32 page_size = 3;
+
+  // The standard list page token.
+  string page_token = 4;
+}
+
+// The response message for [Locations.ListLocations][google.cloud.location.Locations.ListLocations].
+message ListLocationsResponse {
+  // A list of locations that matches the specified filter in the request.
+  repeated Location locations = 1;
+
+  // The standard List next-page token.
+  string next_page_token = 2;
+}
+
+// The request message for [Locations.GetLocation][google.cloud.location.Locations.GetLocation].
+message GetLocationRequest {
+  // Resource name for the location.
+  string name = 1;
+}
+
+// A resource that represents Google Cloud Platform location.
+message Location {
+  // Resource name for the location, which may vary between implementations.
+  // For example: `"projects/example-project/locations/us-east1"`
+  string name = 1;
+
+  // The canonical id for this location. For example: `"us-east1"`.
+  string location_id = 4;
+
+  // The friendly name for this location, typically a nearby city name.
+  // For example, "Tokyo".
+  string display_name = 5;
+
+  // Cross-service attributes for the location. For example
+  //
+  //     {"cloud.googleapis.com/region": "us-east1"}
+  map<string, string> labels = 2;
+
+  // Service-specific metadata. For example the available capacity at the given
+  // location.
+  google.protobuf.Any metadata = 3;
+}

packages/gapic-generator/tests/fragments/test_protobuf_type.proto

@@ -0,0 +1,28 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+package google.fragment;
+
+import "google/api/client.proto";
+import "google/cloud/location/locations.proto";
+
+service MockLocationService {
+  option (google.api.default_host) = "locations.example.com";
+
+  rpc GetLocation(google.cloud.location.GetLocationRequest) returns (google.cloud.location.Location) {
+    option (google.api.method_signature) = "name";
+  }
+}