ref(spans): split load_segment_data into helper steps

[lvthanh03]

Refactors _load_segment_data into an orchestration function with unit testable helpers:

• load payload keys for each segment
• scan/decompress payload batches from those payload keys, return loaded payloads and cleanup keys as LoadedSegmentData

Also updates the cumulative flusher logger to accept loaded segment payloads and build its own FlusherLogEntry values, removing the bookkeeping from flush_segments.

Segment-loss accounting is also split into _record_segment_loss_metrics, so _load_segment_data only loads data.