AXIS2-6104 Null-check getOpMethod, sanitize test data

- All four message receivers (gson + moshi, InOut + InOnly) now check
  for null return from getOpMethod() and throw a clear AxisFault with
  the operation name and service class, instead of NPE
- Replace proprietary department names in pagination test with generic
  "Department A" through "Department O"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>