core: add UUIDv7 context window IDs (#28953)

## Why

The token-budget context currently identifies a context window by its
thread-local sequence number. A UUIDv7 gives the model a stable opaque
identity that remains fixed for a window and rotates when compaction or
`new_context` starts the next one.

## What changed

- Preserve the existing monotonic value as `window_number` and add a
UUIDv7 `window_id` to `CompactedItem`.
- Generate and rotate the UUID with auto-compaction window state,
persist it alongside the number, and reconstruct it on resume and
rollback.
- Accept legacy compacted rollout records where the numeric `window_id`
represented the window number.
- Use the UUID only in token-budget context; existing request headers
and metadata continue using `thread_id:window_number`.

## Testing

- `just test -p codex-protocol compacted_item::tests`
- `just test -p codex-core token_budget`

codex-rs/core/src/compact_remote.rs

@@ -258,7 +258,7 @@ async fn run_remote_compact_task_inner_impl(
             &responses_metadata,
         )
         .await?;
-    let new_window_id = sess.advance_auto_compact_window_id().await;
+    let (new_window_number, new_window_id) = sess.advance_auto_compact_window().await;
     new_history = process_compacted_history(
         sess.as_ref(),
         turn_context.as_ref(),
@@ -274,6 +274,7 @@ async fn run_remote_compact_task_inner_impl(
     let compacted_item = CompactedItem {
         message: String::new(),
         replacement_history: Some(new_history.clone()),
+        window_number: Some(new_window_number),
         window_id: Some(new_window_id),
     };
     // Install is the semantic boundary where the compact endpoint's output becomes live