fix(storage): skip empty coalesce for hot coverage buckets

[fewensa]

Summary

• skip foreground empty-coverage coalescing when a coverage-index-v2 bucket already has more than 128 pending deltas
• append the new empty coverage delta and enqueue the hot bucket for background compaction instead
• keep normal empty coalescing for small buckets

Production evidence

• content-guild-dao Arbitrum query 495015848-495015859 fetched provider rows in 36ms
• the same request then spent the client deadline repeatedly listing hot coverage-index-v2 buckets
• degov split chunks from 25 to 12 to 6 without checkpoint progress

Safety

• does not change read-side max_delta_objects=128
• does not skip coverage publication
• does not delete S3 objects manually
• preserves data correctness: over-budget writes still append coverage; compaction performs later coalescing

Tests

• cargo fmt --check
• cargo test -p datalens-storage coverage_index_v2_empty -- --nocapture
• cargo test -p datalens-storage