fix: coerce None filter_rate in Label/Int Filter charts

[daviddallakyan2005]

Summary

Label Filter and Int Filter Streamlit charts crash with TypeError when a result row has filter_rate=None. dict.get(metric, 0) still returns None when the key is present, so min/sort compare None with a float.

PerformanceCustomDataset never passed filter_rate into the case (unlike LabelFilterPerformanceCase, which sets it to 1.0 - label_percentage). Custom-dataset filter runs therefore land in result files with filter_rate=None.

This PR:

• Coerces missing/None metric values to 0 in both getRange and the x-axis sort (label_filter/charts.py and int_filter/charts.py) so leftover result files still render.
• Sets filter_rate = 1.0 - label_percentage on PerformanceCustomDataset when use_filter and label_percentage are set, matching LabelFilter.

How to test

pip install -e '.[test]'
PYTHONPATH=. python3 -m pytest tests/test_filter_charts.py tests/test_custom_dataset_filter.py -q
make lint

Expected: 8 passed; black --check and ruff check vectordb_bench clean.

To reproduce the crash without the chart-side coerce: open Label Filter (or Int Filter) on a custom-dataset filter result whose case.filter_rate is None.

[sre-ci-robot]

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This pull-request has been approved by: daviddallakyan2005
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