Raise error when transform yields no files

[gordonwatts]

Summary

• raise a ServiceXException when a transform completes successfully but returns no files and provide a dataset identifier in the message
• extend dataset identifier classes to describe themselves so Query reuses those descriptions when building error messages, including file count previews for FileList datasets, and ensure every identifier subclass overrides the helper
• add coverage for the new behaviour and adjust existing dataset tests to avoid empty mocked downloads, including description coverage for CERN Open Data and XRootD identifiers

Testing

• black servicex/query_core.py tests/test_dataset.py servicex/dataset_identifier.py tests/test_dataset_identifier.py
• flake8
• pytest

---

https://chatgpt.com/codex/tasks/task_e_68d5601c52648320850716e40a72cf75

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (56a3e66) to head (69bf7e4).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #662      +/-   ##
==========================================
+ Coverage   97.22%   97.24%   +0.01%     
==========================================
  Files          29       29              
  Lines        1979     1993      +14     
==========================================
+ Hits         1924     1938      +14     
  Misses         55       55              

Flag	Coverage Δ
unittests	97.24% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gordonwatts]

Code will be in #657 .