Allow naming sessions with /new and /clear (#34605)

## What changed

- Accept an optional session name after `/new` or `/clear`.
- Set the requested name through the app server when starting the new thread and update the attached session with that name.
- Report naming failures in the chat while still attaching the newly created thread.

## Testing

- Cover named `/new` and `/clear` dispatch, app-server name persistence, and the existing unnamed `/clear` behavior.

GitOrigin-RevId: 570886abb4af522123f0693ce05d2415e1c0d902

codex-rs/tui/src/slash_command.rs

@@ -155,6 +155,8 @@ impl SlashCommand {
             self,
             SlashCommand::Review
                 | SlashCommand::Rename
+                | SlashCommand::New
+                | SlashCommand::Clear
                 | SlashCommand::Plan
                 | SlashCommand::Goal
                 | SlashCommand::Ide