Improve fallback behavior for partial test windows

[AKKI0511]

Summary

• detect incomplete coverage of configured test windows in time_aware_split and fall back to chronological splits with warnings
• expand integration tests to verify partial-window fallback behavior
• clarify configuration documentation about fallback triggers when coverage is missing

Testing

• poetry run pytest tests/integration/test_pipeline_time_splits.py
• poetry run pre-commit run --all-files

---

Codex Task