## Why
Remote compaction can process large histories. Repeatedly estimating and replacing the full history, and cloning it when tracing is disabled, adds avoidable CPU and memory overhead.
## What changed
- Estimate each history item's token count once, update the total as trailing tool outputs are rewritten, and replace history only after all rewrites are selected.
- Preserve unclamped token totals while calculating removed tokens so saturated estimates do not hide overflow.
- Snapshot compaction input history only when rollout tracing is enabled.
- Reuse the v2 request input instead of cloning it before adding the compaction trigger.
## Testing
- Cover enabled and disabled compaction trace contexts through `is_enabled()` assertions.
GitOrigin-RevId: 8de9c9704ba29532fba05430eb204bbae2e8bf83
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