treewide: add explicit includes for headers losing transitive availability via PCH

Adding Scylla internal headers to the PCH changes which transitive
includes are available. Add explicit includes where needed:
- group0_fwd.hh: timer, lowres_clock, variant, vector
- discovery.hh: unordered_set
- Various test files: result_message.hh, result_set.hh, selection.hh,
  gossiper.hh, seastar net/api and core/seastar headers

service/raft/discovery.hh

@@ -6,6 +6,7 @@
  * SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.1
  */
 #pragma once
+#include <unordered_set>
 #include "service/raft/group0_fwd.hh"
 
 namespace service {

service/raft/group0_fwd.hh

@@ -9,6 +9,10 @@
 #pragma once
 
 #include <iosfwd>
+#include <variant>
+#include <vector>
+#include <seastar/core/timer.hh>
+#include <seastar/core/lowres_clock.hh>
 #include "raft/raft_fwd.hh"
 #include "gms/inet_address.hh"
 

test/boost/commitlog_cleanup_test.cc

@@ -11,6 +11,7 @@
 #undef SEASTAR_TESTING_MAIN
 #include <seastar/testing/test_case.hh>
 #include "test/lib/cql_test_env.hh"
+#include "transport/messages/result_message.hh"
 #include "db/commitlog/commitlog_replayer.hh"
 #include "db/commitlog/commitlog.hh"
 #include "db/config.hh"

test/boost/per_partition_rate_limit_test.cc

@@ -9,6 +9,7 @@
 
 #include "mutation/mutation.hh"
 #include "service/storage_proxy.hh"
+#include "cql3/selection/selection.hh"
 
 BOOST_AUTO_TEST_SUITE(per_partition_rate_limit_test)
 

test/boost/schema_change_test.cc

@@ -18,6 +18,8 @@
 
 #include "test/lib/cql_test_env.hh"
 #include "test/lib/cql_assertions.hh"
+#include "transport/messages/result_message.hh"
+#include "cql3/result_set.hh"
 #include "service/migration_manager.hh"
 #include "service/storage_proxy.hh"
 #include "schema/schema_builder.hh"

test/boost/tablets_test.cc

@@ -18,6 +18,7 @@
 #include "test/lib/random_utils.hh"
 #include "service/topology_mutation.hh"
 #include "service/storage_service.hh"
+#include "gms/gossiper.hh"
 #include <fmt/ranges.h>
 #include <seastar/testing/thread_test_case.hh>
 #include <seastar/testing/on_internal_error.hh>

test/perf/perf_generic_server.cc

@@ -11,6 +11,8 @@
 #include <seastar/coroutine/parallel_for_each.hh>
 #include <seastar/core/temporary_buffer.hh>
 #include <seastar/net/socket_defs.hh>
+#include <seastar/net/api.hh>
+#include <seastar/core/seastar.hh>
 #include <seastar/core/future.hh>
 #include <seastar/core/shared_ptr.hh>
 #include <seastar/core/thread.hh>

test/vector_search/rescoring_test.cc

@@ -14,6 +14,7 @@
 #include "vector_search/vector_store_client.hh"
 #include "vs_mock_server.hh"
 #include "test/lib/cql_test_env.hh"
+#include "transport/messages/result_message.hh"
 #include "utils/rjson.hh"
 #include <boost/test/tools/old/interface.hpp>
 #include <seastar/core/seastar.hh>