Fixes #7974

Summary

This PR prevents a panic (alloc::raw_vec::capacity_overflow) in
segment::index::query_estimator::combine_min_should_estimations when
min_should.min_count is an edge value.

What changed

• Add guards in combine_min_should_estimations:
  • min_count == 0 → CardinalityEstimation::exact(total) (trivially satisfied)
  • min_count > estimations.len() → CardinalityEstimation::exact(0) (unsatisfiable)
• Add 2 regression unit tests covering both cases.

Why

Without these guards, malformed/edge inputs can reach combinations(min_count) and
trigger pathological allocation, leading to a server panic instead of a safe estimate.

Local verification

cargo test -p segment ✅

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