test: validate diff exit codes in issue-2271

Agent-Logs-Url: https://github.com/helmfile/helmfile/sessions/5615c543-f96b-44ed-be25-ca1559ee6ab0

Co-authored-by: yxxhero <11087727+yxxhero@users.noreply.github.com>

test/integration/test-cases/issue-2271.sh

@@ -40,12 +40,19 @@ assert_lookup_preserved() {
   local label="$1"
   local helmfile_path="$2"
   local output_path="$3"
+  local code
 
   info "Testing diff with ${label} - lookup should preserve value"
 
   ${helmfile} -f "${helmfile_path}" diff > "${output_path}" 2>&1
   code=$?
 
+  if [ $code -ne 0 ] && [ $code -ne 2 ]; then
+    cat "${output_path}"
+    rm -rf "${issue_2271_tmp_dir}"
+    fail "Unexpected error during diff with ${label}"
+  fi
+
   # Check if the diff contains the preserved value (not "initial-value")
   if grep -q "preserved-value.*test-preserved-value" "${output_path}"; then
     info "SUCCESS: lookup function preserved the value with ${label}"