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/src/codex_message_processor.rs

@@ -183,6 +183,7 @@ use codex_core::AuthManager;
 use codex_core::CodexAuth;
 use codex_core::CodexThread;
 use codex_core::Cursor as RolloutCursor;
+use codex_core::ForkSnapshot;
 use codex_core::NewThread;
 use codex_core::RolloutRecorder;
 use codex_core::SessionMeta;
@@ -4039,7 +4040,7 @@ impl CodexMessageProcessor {
         } = match self
             .thread_manager
             .fork_thread(
-                usize::MAX,
+                ForkSnapshot::Interrupted,
                 config,
                 rollout_path.clone(),
                 persist_extended_history,
@@ -6508,7 +6509,7 @@ impl CodexMessageProcessor {
         } = self
             .thread_manager
             .fork_thread(
-                usize::MAX,
+                ForkSnapshot::Interrupted,
                 config,
                 rollout_path,
                 /*persist_extended_history*/ false,