varlink: add enum types for scheduling and mount settings in io.systemd.Unit

Convert CPUSchedulingPolicy, IOSchedulingClass, NUMAPolicy and MountFlags
fields from plain strings to proper varlink enum types in the io.systemd.Unit
interface. Update the corresponding serialization code to use
json_underscorify() for correct enum value formatting.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

src/test/test-varlink-idl-unit.c

@@ -1,6 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include "cgroup.h"
+#include "ioprio-util.h"
+#include "numa-util.h"
+#include "process-util.h"
 #include "tests.h"
 #include "test-varlink-idl-util.h"
 #include "unit.h"
@@ -26,6 +29,17 @@ TEST(unit_enums_idl) {
         TEST_IDL_ENUM(PrivatePIDs, private_pids, vl_type_PrivatePIDs);
         TEST_IDL_ENUM(PrivateBPF, private_bpf, vl_type_PrivateBPF);
 
+        /* sched_policy table has gaps (SCHED_IDLE=5, SCHED_EXT=7), so only test from_string direction */
+        TEST_IDL_ENUM_FROM_STRING(int, sched_policy, vl_type_CPUSchedulingPolicy);
+        /* ioprio_class uses _alloc variant for to_string, so only test from_string direction */
+        TEST_IDL_ENUM_FROM_STRING(int, ioprio_class, vl_type_IOSchedulingClass);
+        TEST_IDL_ENUM(int, mpol, vl_type_NUMAPolicy);
+
+        /* mount_propagation_flag has non-standard from_string API, test manually */
+        test_enum_to_string_name("shared", &vl_type_MountPropagationFlag);
+        test_enum_to_string_name("slave", &vl_type_MountPropagationFlag);
+        test_enum_to_string_name("private", &vl_type_MountPropagationFlag);
+
         /* CGroupContext enums */
         TEST_IDL_ENUM(CGroupDevicePolicy, cgroup_device_policy, vl_type_CGroupDevicePolicy);
         TEST_IDL_ENUM(ManagedOOMMode, managed_oom_mode, vl_type_ManagedOOMMode);