chore: add cargo fmt check to Rust CI and fix existing violations (#620)

TypeScript CI has prettier --check but Rust CI only runs cargo test.
Add cargo fmt --check to catch formatting issues early, and fix the
8 pre-existing formatting violations on main.

.github/workflows/ci.yml

@@ -64,12 +64,17 @@ jobs:
 
       - name: Setup Rust toolchain
         uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
 
       - name: Cache Rust build artifacts
         uses: Swatinem/rust-cache@v2
         with:
           workspaces: cli
 
+      - name: Format check
+        run: cargo fmt --manifest-path cli/Cargo.toml -- --check
+
       - name: Run Rust tests
         run: cargo test --profile ci --manifest-path cli/Cargo.toml