Allow naming forked chats from the TUI (#36036)

## What changed

- Accept an optional thread name with `/fork`, trimming surrounding whitespace.
- Apply the name to the newly forked thread and update the active session metadata.
- Keep the successful fork active and show an error when naming it fails.

## Testing

- Cover unnamed and named `/fork` dispatch, persisted names, and naming failures.

GitOrigin-RevId: 1799576f82ef0856f992a0378f041e4bab89621b

codex-rs/tui/src/slash_command.rs

@@ -157,6 +157,7 @@ impl SlashCommand {
                 | SlashCommand::Rename
                 | SlashCommand::New
                 | SlashCommand::Clear
+                | SlashCommand::Fork
                 | SlashCommand::Plan
                 | SlashCommand::Goal
                 | SlashCommand::Ide