Keep side conversations open when switching threads (#35011)

## What changed

- Add a configurable `toggle_side_conversation` TUI action, bound to `ctrl-/` by default, to switch between a side conversation and its parent without closing either.
- Show the active binding in the footer and distinguish switching from closing with `ctrl-c`.
- Allow starting a replacement side conversation from the parent, while cleaning up the previous side thread and preserving it if cleanup fails.
- Clean up retained side conversations when shutting down the current thread.

## Testing

- Cover shortcut remapping, conflicts, and compatibility with existing `ctrl-/` and `ctrl-7` bindings.
- Cover side-thread replacement and footer states for both the parent and side conversation.

GitOrigin-RevId: a621c21e1c45ba73197d59750b2e05efa5ee1081

codex-rs/tui/src/bottom_pane/footer.rs

@@ -593,7 +593,7 @@ pub(crate) fn side_conversation_context_line(label: &str) -> Line<'static> {
     if let Some(rest) = label.strip_prefix("Side ") {
         Line::from(vec!["Side".magenta().bold(), format!(" {rest}").magenta()])
     } else {
-        Line::from(label.to_string()).magenta()
+        Line::from(vec![Span::from(label.to_string()).magenta()])
     }
 }