[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/core/src/state/session_tests.rs

@@ -65,18 +65,16 @@ async fn set_rate_limits_defaults_limit_id_to_codex_when_missing() {
 }
 
 #[tokio::test]
-async fn replace_history_clears_auto_compact_window_prefill_without_advancing() {
+async fn replace_history_clears_auto_compact_window_prefill() {
     let session_configuration = make_session_configuration_for_tests().await;
     let mut state = SessionState::new(session_configuration);
 
-    state.start_next_auto_compact_window();
     state.set_auto_compact_window_estimated_prefill(/*tokens*/ 100);
     state.replace_history(Vec::new(), /*reference_context_item*/ None);
 
     assert_eq!(
         state.auto_compact_window_snapshot(),
         AutoCompactWindowSnapshot {
-            ordinal: 2,
             prefill_input_tokens: None,
         }
     );