fix(presentation): dedupe duplicate main metric columns

[MukundaKatta]

Summary

• dedupe substituted metric matchers in create_group_table so a scenario main metric does not render twice when it also appears in another metric group
• add a regression test covering main_metric plus an explicit accuracy group resolving to the same metric column

Testing

• python3 -m compileall src/helm/benchmark/presentation/summarize.py src/helm/benchmark/presentation/test_summarize.py
• tried PYTHONPATH=/tmp/helm-user/src python3 -m pytest /tmp/helm-user/src/helm/benchmark/presentation/test_summarize.py -k deduplicates_main_metric_columns
• local pytest startup was blocked because this environment is missing cattrs (ModuleNotFoundError: No module named 'cattrs')