Add fork snapshot modes (#15239)

## Summary
- add `ForkSnapshotMode` to `ThreadManager::fork_thread` so callers can
request either a committed snapshot or an interrupted snapshot
- share the model-visible `<turn_aborted>` history marker between the
live interrupt path and interrupted forks
- update the small set of direct fork callsites to pass
`ForkSnapshotMode::Committed`

Note: this enables /btw to work similarly as Esc to interrupt (hopefully
somewhat in distribution)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/tests/suite/v2/thread_fork.rs

@@ -123,7 +123,7 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
         "expected forked thread to include one turn"
     );
     let turn = &thread.turns[0];
-    assert_eq!(turn.status, TurnStatus::Completed);
+    assert_eq!(turn.status, TurnStatus::Interrupted);
     assert_eq!(turn.items.len(), 1, "expected user message item");
     match &turn.items[0] {
         ThreadItem::UserMessage { content, .. } => {