fix(workflows): address Copilot review round 10 comments

- shared-release: add file existence check for makefile-path in
  Validate inputs step to fail fast with a clear message instead of
  a confusing grep error when the path is wrong

Signed-off-by: I313226 <onur.yilmaz@sap.com>

Author: onuryilmaz

.github/workflows/shared-release.yaml

@@ -82,6 +82,10 @@ jobs:
             echo "ERROR: makefile-path is required for version computation"
             exit 1
           fi
+          if [ ! -f "${{ inputs.makefile-path }}" ]; then
+            echo "ERROR: makefile-path '${{ inputs.makefile-path }}' does not exist"
+            exit 1
+          fi
 
       - name: Compute new version
         id: version