fix(results): tag unsupported failure explanations

[wolfiesch]

Summary

• add the missing unsupported tag when constructing FailureExplanation for unsupported diagnostics
• fixes post-merge master mypy and benchmark rendering failures from feat(results): separate unsupported vs failed, add run deltas, and surface failure tags #34

Verification

• uv run mypy src/excelbench/results/failure_explainer.py
• uv run pytest tests/test_failure_explainer.py tests/test_results_html_dashboard.py tests/test_dashboard.py
• uv run excelbench benchmark --tests fixtures/excel --output /tmp/excelbench-results-postmerge-fix

[Copilot]

Pull request overview

Ensures FailureExplanation objects created for UNSUPPORTED_FEATURE diagnostics include the tag field, preventing type-check/rendering errors and correctly labeling unsupported outcomes in report/dashboard layers.

Changes:

• Add tag="unsupported" when constructing FailureExplanation for DiagnosticCategory.UNSUPPORTED_FEATURE.

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