add additional input validation

Author: rinej

action.yml

@@ -76,6 +76,12 @@ runs:
             exit 1
           fi
 
+          # Validate keystore file actually exists if provided
+          if [ -n "${{ inputs.keystore-file }}" ] && [ ! -f "${{ inputs.keystore-file }}" ]; then
+            echo "Keystore file '${{ inputs.keystore-file }}' does not exist."
+            exit 1
+          fi
+
           if [ -z "${{ inputs.keystore-store-file }}" ]; then
             echo "  Input 'keystore-store-file' is required for signed builds."
             exit 1