Apply Gemini review: error fallback, doc return types, get_instance note

- Change _invoke_json fallback from 200 "Service loaded" to 500 error
  response with LOG_ERROR — prevents masking misconfigured services
- Add explicit return types (axis2_char_t*) to _process_json_only
  signature in HTTP2_ANDROID.md
- Add note to HTTP2_JSON_DEBUG.md Problem 5 that services can avoid the
  axis2_get_instance signature trap by not exporting it at all (like
  bigdata-h2-service, login-service, testws-service)

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