varlink: add enum types for configuration settings in io.systemd.Manager

Convert 8 string fields in the io.systemd.Manager varlink interface to
proper enum types:

- LogTarget: new enum (console, console_prefixed, kmsg, journal, ...)
- DefaultStandardOutput/Error: reuse ExecOutputType from common
- DefaultMemory/CPU/IOPressureWatch: reuse CGroupPressureWatch from common
- DefaultOOMPolicy: new enum (continue, stop, kill)
- CtrlAltDelBurstAction: reuse EmergencyAction from common

Output serialization updated to use JSON_BUILD_PAIR_ENUM for automatic
underscorification of dash-containing values.

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

src/test/meson.build

@@ -500,6 +500,9 @@ executables += [
         core_test_template + {
                 'sources' : files('test-varlink-idl-unit.c'),
         },
+        core_test_template + {
+                'sources' : files('test-varlink-idl-manager.c'),
+        },
         test_template + {
                 'sources' : files('test-watchdog.c'),
                 'type' : 'unsafe',

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

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "cgroup.h"
+#include "emergency-action.h"
+#include "execute.h"
+#include "log.h"
+#include "tests.h"
+#include "test-varlink-idl-util.h"
+#include "unit.h"
+#include "varlink-idl-common.h"
+#include "varlink-io.systemd.Manager.h"
+
+TEST(manager_enums_idl) {
+        /* ManagerContext enums */
+        TEST_IDL_ENUM(LogTarget, log_target, vl_type_LogTarget);
+        TEST_IDL_ENUM(OOMPolicy, oom_policy, vl_type_OOMPolicy);
+
+        /* ExecOutput values like "kmsg+console" contain '+' which becomes '_' via underscorify,
+         * but dashify won't restore it, so from_string round-trip fails. Test to_string direction only. */
+        TEST_IDL_ENUM_TO_STRING(ExecOutput, exec_output, vl_type_ExecOutputType);
+        TEST_IDL_ENUM(CGroupPressureWatch, cgroup_pressure_watch, vl_type_CGroupPressureWatch);
+        TEST_IDL_ENUM(EmergencyAction, emergency_action, vl_type_EmergencyAction);
+}
+
+DEFINE_TEST_MAIN(LOG_DEBUG);