## Why
Compaction analytics need token counts that better represent the request
being compacted. The existing session snapshot can diverge from the
actual remote compaction request after output rewriting, and remote v2
can use server-side Responses usage when available.
## What changed
- Add an optional `active_context_tokens_before` override to
`CompactionAnalyticsAttempt::track(...)` for remote compaction when it
has a better before-token value than the begin-time session snapshot.
The local `/compact` path passes no override.
- For remote v1 `responses_compact`, subtract the estimated token delta
from pre-compaction output rewriting from the session snapshot, capped
by locally-added tokens since the last successful API response.
- For remote v2 `responses_compaction_v2`, use the same bounded
output-rewrite fallback as remote v1, then overwrite
`active_context_tokens_before` with server `token_usage.input_tokens`
from the `response.completed` event when present.
- Keep the existing v2 compaction-output validation while carrying the
completed response token usage through `collect_compaction_output`.
## Verification
- `just fmt`
- `just test -p codex-core
collect_compaction_output_accepts_additional_output_items`
- `git diff --check`
rhan-oai
c143a86de8