Validate output artifact file paths

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Summary

• reject output artifact paths that already exist as directories before writing tables, JSON, or plots
• reuse the output path guard across table/summary writes and plot rendering
• cover run and plot CLI failures for directory-valued artifact paths without tracebacks

Tests

• pytest tests/test_cli_errors.py tests/test_pipeline_e2e.py
• pytest
• python -m telemetry_window_demo.cli run --config configs/default.yaml