Harden OpenNeuro MEG staging and PCA decoding

[FlorianPfaff]

Summary

• filter ds004276 staging to auditory word event rows before joining the behavior file
• drop events whose requested epoch window falls outside raw-file bounds before applying trial caps
• make integer PCA component requests fold-safe while preserving requested provenance and pipeline naming
• document OpenNeuro workflow caching/API-key behavior and dataset-specific hardening

Tests

• PYTHONPATH=src python -m pytest tests/test_openneuro_meg.py tests/test_decoding.py --basetemp .pytest-tmp
• PYTHONPATH=src python -m pytest tests/test_mne_time_decode.py --basetemp .pytest-tmp
• PYTHONPATH=src python -m ruff check src/neureptrace/openneuro_meg.py src/neureptrace/decoding/__init__.py tests/test_openneuro_meg.py tests/test_decoding.py

[FlorianPfaff]

Local validation passed. I also attempted to dispatch openneuro-meg-loso.yml smoke runs for ds004276, ds006629, and ds004330 on this branch, but GitHub returned HTTP 500 Failed to run workflow dispatch for both gh workflow run and the REST dispatch endpoint. No runs were created for this branch.