Session picker shows thread_name if set (#10340)

- shows names of threads in the ResumePicker used by `/resume` and
`codex resume` if set, default to preview (previous behaviour) if none
- adds a `find_thread_names_by_ids` that maps names to IDs in
`codex-rs/core/src/rollout/session_index.rs`. It reads sequentially in
normal (instead of reverse order in `codex resume <name>`) the index
mapping file. This function is called from a list of session (default
page is 25, pages loaded depends of height of terminal), for which most
of them will always have at least one session unnamed and require the
whole file to be read therefore. Could be better and sqlite integration
will make this better
- those reads won't be needed when leveraging sqlite
 

Opened questions:
- We could rename the TUI "Conversation" column to "Name" or "Thread"
that would feel more accurate. Could be a fast-follow if we implement
auto-naming as it'll always be a name instead?

codex-rs/core/src/lib.rs

@@ -120,6 +120,7 @@ pub use rollout::list::parse_cursor;
 pub use rollout::list::read_head_for_summary;
 pub use rollout::list::read_session_meta_line;
 pub use rollout::rollout_date_parts;
+pub use rollout::session_index::find_thread_names_by_ids;
 pub use transport_manager::TransportManager;
 mod function_tool;
 mod state;