refactor: extract context window token status (#29664)

## 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`













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* #29665
* __->__ #29664

codex-rs/core/src/session/mod.rs

@@ -211,6 +211,7 @@ use codex_protocol::exec_output::StreamOutput;
 
 mod code_mode_warning;
 mod config_lock;
+pub(crate) mod context_window;
 mod handlers;
 mod inject;
 mod input_queue;