feat: fork conversation/thread (#8866)

## Summary
- add thread/conversation fork endpoints to the protocol (v1 + v2)
- implement fork handling in app-server using thread manager and config
overrides
- add fork coverage in app-server tests and document `thread/fork` usage

codex-rs/app-server/tests/suite/mod.rs

@@ -3,6 +3,7 @@ mod auth;
 mod codex_message_processor_flow;
 mod config;
 mod create_thread;
+mod fork_thread;
 mod fuzzy_file_search;
 mod interrupt;
 mod list_resume;