Man, this is such a specific and cool request. I immediately think of 'The Wolf and the Woodsman' by Ava Reid, though the celestial element there is maybe more woven into the broader mythology than being a straight-up labeled protector? The wolf god aspects feel ancient and cosmic. Also, check out 'The Last Wolf' from Maria Vale's 'The Legend of All Wolves' series—it's deeply shifter-focused, but the pack's connection to the moon and ancient pacts gives it a celestial guardian vibe. I'm pretty sure there's a middle-grade series, maybe 'The Familiars'? No, that's owls. Wait, I remember a Webnovel on RoyalRoad called 'Beneath the Dragoneye Moons' where a lunar wolf spirit becomes a companion, but that's more progression fantasy. Honestly, the 'celestial wolf as mystical protector' feels like a trope that lives more in fanfic and indie fantasy than mainstream publishing, which is a shame because it's an awesome image.
If you're open to manga, 'Wolf's Rain' has a heavy celestial/almost biblical destiny angle with the wolves seeking paradise. They're definitely protectors of a sort, guarding the hope of the world. For a more direct match, you might have to dig into Kindle Unlimited's paranormal romance sections—look for shifter books with 'moon,' 'star,' or 'celestial' in the title. There's one called 'Star-Crossed Wolves' that might fit the bill, but I haven't read it.