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/app-server-protocol/src/protocol/thread_history.rs

@@ -3340,6 +3340,7 @@ mod tests {
             RolloutItem::Compacted(CompactedItem {
                 message: String::new(),
                 replacement_history: None,
+                window_number: None,
                 window_id: None,
             }),
             RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {