docs: add comprehensive query and read results sample

google/cloud/bigquery_unified/samples/query_and_read_results.cc

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+// 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
+//
+//     https://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.
+
+// This sample demonstrates how to execute a BigQuery query using the
+// insert+poll workflow and how to read query results using futures
+// and callbacks.
+
+#include "google/cloud/bigquery_unified/client.h"
+
+#include <future>
+#include <iostream>
+#include <string>
+
+namespace bigquery = ::google::cloud::bigquery_unified;
+
+int main(int argc, char* argv[]) {
+  if (argc != 3) {
+    std::cerr << "Usage: " << argv[0]
+              << " <project-id> <dataset-id>\n";
+    return 1;
+  }
+
+  std::string project_id = argv[1];
+  std::string dataset_id = argv[2];
+
+  // Create a BigQuery client using default credentials (creating connection with google cloud-secure and standard)
+  auto client =
+      bigquery::Client(bigquery::MakeBigQueryConnection());
+
+  // A simple, read-only query suitable for samples.
+  std::string query =
+      "SELECT table_name FROM `" + project_id + "." + dataset_id +
+      ".INFORMATION_SCHEMA.TABLES` LIMIT 10";
+
+  // Execute the query using the insert+poll workflow.
+  auto query_future = client.InsertQueryAndPoll(query);
+
+  // Attach a continuation to process the results once the query completes.
+  auto result_future = query_future.then(
+      [](auto f) {
+        auto response = f.get();
+        if (!response) {
+          std::cerr << "Query failed: "
+                    << response.status() << "\n";
+          return;
+        }
+
+        // Iterate over rows and fields in the result set.
+        for (auto const& row : response->rows) {
+          for (auto const& field : row) {
+            std::cout << field.value << " ";
+          }
+          std::cout << "\n";
+        }
+      });
+
+  // Wait for the callback to finish.
+  result_future.get();
+
+  return 0;
+}