Allow ThreadManager to customize thread ID generation (#37350)

## What changed

- Add `ThreadManager::with_thread_id_generator` to configure ID allocation for new root, child, and forked threads.
- Keep the standard UUIDv7 generator as the default and preserve stored IDs when resuming threads.
- Add `ThreadId::from_u128` for constructing identifiers from UUID-compatible values.

## Testing

- Cover default allocation, custom IDs across roots, child agents, and forks, and ID preservation on resume.

GitOrigin-RevId: 6c38c862e30a50bad944a994bcbccf7d1e7ada24

codex-rs/core/src/session/session.rs

@@ -569,7 +569,7 @@ impl Session {
 
         let thread_id = match &initial_history {
             InitialHistory::New | InitialHistory::Cleared | InitialHistory::Forked(_) => {
-                ThreadId::default()
+                agent_control.generate_thread_id()
             }
             InitialHistory::Resumed(resumed_history) => resumed_history.conversation_id,
         };