[codex] Store compact window id in rollout (#27264)

## Why

Compaction window identity is part of session history, not model-client
transport state. Persisting it with the compacted rollout item lets
resumed threads continue from the reconstructed window without keeping
mutable window state on `ModelClient`.

## What changed

- Added `window_id` to `CompactedItem` and stamp it when
`replace_compacted_history` installs compacted history.
- Moved auto-compact window id ownership into `AutoCompactWindow` /
`SessionState`; `ModelClient` now receives the request window id from
callers instead of storing it.
- Returned `window_id` from rollout reconstruction for resume.
Reconstruction uses the newest surviving compacted item's stored
`window_id` when present, and falls back to the legacy compacted-item
count when it is absent.
- Kept fork startup at the fresh default window id and updated direct
model-client tests to pass explicit test window ids.

## Validation

- `cargo check -p codex-core --tests`

codex-rs/thread-store/src/thread_metadata_sync.rs

@@ -474,6 +474,7 @@ mod tests {
         let item = RolloutItem::Compacted(CompactedItem {
             message: "compacted".to_string(),
             replacement_history: None,
+            window_id: None,
         });
 
         let first = sync