shared: fix sysext ImageType IDL missing mstack value, add enum IDL test

Add the missing 'mstack' value to the ImageType enum in the
io.systemd.sysext Varlink IDL — image_type_table in discover-image.c
already defines IMAGE_MSTACK = "mstack" but it was absent from the IDL.

Export vl_type_ImageType so the test can reference it, and add
TEST_IDL_ENUM(ImageType, ...) to the enums_idl test to catch
similar regressions in the future.

src/shared/varlink-io.systemd.sysext.c

@@ -8,12 +8,13 @@ static SD_VARLINK_DEFINE_ENUM_TYPE(
                 SD_VARLINK_DEFINE_ENUM_VALUE(sysext),
                 SD_VARLINK_DEFINE_ENUM_VALUE(confext));
 
-static SD_VARLINK_DEFINE_ENUM_TYPE(
+SD_VARLINK_DEFINE_ENUM_TYPE(
                 ImageType,
                 SD_VARLINK_DEFINE_ENUM_VALUE(directory),
                 SD_VARLINK_DEFINE_ENUM_VALUE(subvolume),
                 SD_VARLINK_DEFINE_ENUM_VALUE(raw),
-                SD_VARLINK_DEFINE_ENUM_VALUE(block));
+                SD_VARLINK_DEFINE_ENUM_VALUE(block),
+                SD_VARLINK_DEFINE_ENUM_VALUE(mstack));
 
 static SD_VARLINK_DEFINE_METHOD(
                 Merge,

src/shared/varlink-io.systemd.sysext.h

@@ -3,4 +3,5 @@
 
 #include "sd-varlink-idl.h"
 
+extern const sd_varlink_symbol vl_type_ImageType;
 extern const sd_varlink_interface vl_interface_io_systemd_sysext;

src/test/test-varlink-idl.c

@@ -8,6 +8,7 @@
 #include "sd-varlink-idl.h"
 
 #include "bootspec.h"
+#include "discover-image.h"
 #include "fd-util.h"
 #include "gpt.h"
 #include "json-util.h"
@@ -541,6 +542,9 @@ TEST(enums_idl) {
 
         TEST_IDL_ENUM(DnsOverTlsMode, dns_over_tls_mode, vl_type_DNSOverTLSMode);
         TEST_IDL_ENUM(ResolveSupport, resolve_support, vl_type_ResolveSupport);
+
+        TEST_IDL_ENUM(ImageType, image_type, vl_type_ImageType);
+        TEST_IDL_ENUM_TO_STRING(ImageType, image_type, vl_type_ImageType);
 }
 
 static SD_VARLINK_DEFINE_METHOD(