fix(query-engine): reject sum-weighted heap CMS for COUNT matching #666

This was generated by AI during triage.*

Problem

Capability matching currently allows CountMinSketchWithHeap to satisfy ordinary Statistic::Count requirements without checking its count_events weighting. A heap configured with count_events: false is value-weighted (SUM semantics), but can therefore be selected for COUNT and return summed values.

Expected behavior

Ordinary COUNT matching must reject heap sketches with count_events: false and accept sketches whose count_events is true or omitted (the existing default). Existing Top-k weighting behavior must remain unchanged.

Acceptance criteria

• Add behavioral capability-matching tests covering both heap weighting variants.
• Ensure a SUM-weighted heap cannot serve ordinary COUNT.
• Preserve existing Top-k matching semantics.
• Implement test-first (TDD).

Found during review of #665, Roborev job #124 finding 4.