[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/protocol/src/protocol.rs

@@ -2892,6 +2892,8 @@ pub struct CompactedItem {
     pub message: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub replacement_history: Option<Vec<ResponseItem>>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub window_id: Option<u64>,
 }
 
 impl From<CompactedItem> for ResponseItem {