Include source_id and variant_label in batch report

[rbeucher]

Summary

• Add source_id and variant_label to the top-level moppy_batch_report.json output, sourced from batch_config.yml the same way experiment_id already is.
• This lets downstream tooling (e.g. the access-moppy-progress dashboard registry's manage-submission command) resolve model/member identifiers directly from the report instead of requiring them to be typed manually on every submission.

Test plan

• pixi run -e test pytest tests/unit/test_batch_report.py -q — 27 passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.1%. Comparing base (16227e6) to head (3590fff).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #605   +/-   ##
=====================================
  Coverage   75.1%   75.1%           
=====================================
  Files         38      38           
  Lines       8070    8070           
  Branches    1517    1517           
=====================================
  Hits        6061    6061           
  Misses      1688    1688           
  Partials     321     321           

Flag	Coverage Δ
unit	75.1% <ø> (ø)

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