Add DLL destructor to clean up handles on unload

When the .NET test host exits, it unloads the DLL without calling
MsiCloseAllHandles. This leaves GLib objects in a partially-freed
state, causing g_object_unref crashes during GLib's atexit cleanup.

Add a __attribute__((destructor)) that calls handle_table_close_all()
and gsf_shutdown() to properly release all handles and shut down
libgsf before the DLL is unloaded.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: caesay

msi-interop/handle_table.c

@@ -284,3 +284,11 @@ handle_table_auto_init(void)
     g_type_ensure(libmsi_summary_info_get_type());
     handle_table_init();
 }
+
+__attribute__((destructor))
+static void
+handle_table_auto_cleanup(void)
+{
+    handle_table_close_all();
+    gsf_shutdown();
+}