CI : installer rustfmt pour cargo fmt --check

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: 1.98.1
-          components: clippy
+          components: clippy, rustfmt
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: server