[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/client_tests.rs

@@ -291,13 +291,13 @@ fn build_ws_client_metadata_includes_window_lineage_and_turn_metadata() {
         Some(parent_thread_id),
     );
 
-    client.advance_window_generation();
-
+    let thread_id = client.state.thread_id;
+    let window_id = format!("{thread_id}:1");
     let client_metadata = client.build_ws_client_metadata(
+        &window_id,
         Some(r#"{"turn_id":"turn-123"}"#),
         /*use_responses_lite*/ false,
     );
-    let thread_id = client.state.thread_id;
     assert_eq!(
         client_metadata,
         std::collections::HashMap::from([