ZugferdKositValidator -> Made message bag type constants private #193

All 1 852 tests pass in 5s

1 852 tests ±0 1 852 ✔️ ±0 5s ⏱️ ±0s
      1 suites ±0       0 💤 ±0
      1 files ±0       0 ❌ ±0

Results for commit bfa1560. ± Comparison against earlier commit ac81ad1.