Add scalafmtCheckAll to code format CI (msgpack#902)

This ensures Scala code formatting is validated in CI alongside Java checkstyle.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: xerial

.github/workflows/CI.yml

@@ -42,6 +42,8 @@ jobs:
       - uses: actions/checkout@v4
       - name: jcheckstyle
         run: ./sbt jcheckStyle
+      - name: scalafmtCheckAll
+        run: ./sbt scalafmtCheckAll
 
   test:
     name: Test JDK${{ matrix.java }}