fix: flaky test test_ann_prefilter for HNSW

[BubbleCal]

this test searches for top1 which could lead to greedy search for HNSW, fix it by setting ef

[github-actions]

PR Review

Summary: This PR adds scan.ef(100) to fix a flaky test test_ann_prefilter for HNSW index.

Assessment

The fix is reasonable. The ef parameter controls the search exploration factor in HNSW - a higher value increases recall at the cost of search speed. For a test that's checking correctness of prefiltering behavior (not performance), setting ef(100) ensures the search is thorough enough to reliably find the correct result.

No Major Issues Found

This is a minimal, targeted fix that:

• Only affects test code
• Uses an established API (ef()) that's already defined in the scanner
• Is consistent with the existing minimum_nprobes(100) already set for IVF indices

LGTM ✓

[Xuanwo]

Thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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