ci: add clippy check to Rust CI workflow (#675)

Add `cargo clippy -- -D warnings` step to the Rust CI job so that
clippy warnings fail the build. Also fix the one new lint
(`unnecessary_map_or`) introduced in the current stable clippy.

Fixes #672

Co-authored-by: hyunjinee <leehj0110@kakao.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -65,7 +65,7 @@ jobs:
       - name: Setup Rust toolchain
         uses: dtolnay/rust-toolchain@stable
         with:
-          components: rustfmt
+          components: rustfmt, clippy
 
       - name: Cache Rust build artifacts
         uses: Swatinem/rust-cache@v2
@@ -75,6 +75,9 @@ jobs:
       - name: Format check
         run: cargo fmt --manifest-path cli/Cargo.toml -- --check
 
+      - name: Clippy check
+        run: cargo clippy --manifest-path cli/Cargo.toml -- -D warnings
+
       - name: Run Rust tests
         run: cargo test --profile ci --manifest-path cli/Cargo.toml