uSync blueprint bulk-save fix, backported to v17

Repo: KevinJump/uSync

Merged #1035: ContentTemplateSerializer now saves blueprints via SaveBlueprint on the bulk save path too, not just the single-item path — the bulk path was persisting through IContentService.Save, which rewrote a blueprint's node object type to Document and broke re-imports with a duplicate key error
Backported the fix to v17/main in #1037, referencing #1034