## Why
This PR keeps the mechanical helper extraction separate from the
behavior change in #29665. The follow-up needs the token-window
accounting from `turn.rs` in another call path, but reviewing that is
much easier when the helper extraction is separate from the semantic
change.
## What
- Adds `session/context_window.rs` with `ContextWindowTokenStatus`.
- Moves the existing auto-compaction token-status calculation out of
`session/turn.rs`.
- Replaces the duplicated inline remaining-token calculation in
`turn.rs` with `tokens_until_compaction()`.
This PR is intended to be behavior-preserving. The
`get_context_remaining` behavior change is stacked separately in #29665.
## Testing
- `just test -p codex-core auto_compact_body_after_prefix`
---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/29664).
* #29665
* __->__ #29664
Michael BolinandGitHub
4dde907d27