Fix checkstyle execution to use Maven validate phase

- Changed from direct checkstyle:check with manual config to validate phase
- This ensures proper resolution of ${basedir} property in checkstyle config
- Uses the checkstyle execution already defined in pom.xml (lines 535-555)

Author: shahar-biron

.github/workflows/ci.yml

@@ -135,4 +135,4 @@ jobs:
           ${{ runner.os }}-maven-
           
     - name: Run Checkstyle
-      run: ./mvnw checkstyle:check -B -Dcheckstyle.config.location=etc/checkstyle/config.xml -Dcheckstyle.suppressions.location=etc/checkstyle/suppressions.xml
+      run: ./mvnw validate -B